How Was the Order of the Quran Determined?
Question

Why were not the surahs of the Quran ordered according to the order of revelation?
Praise be to Allah.Order of verses in one Surah
The fact there is scholarly consensus, and many texts stating the order of verses in one Surah, that is very well known. The consensus was reported by many scholars, including Al-Zarkashi in al-Burhan and Abu Ja’far, who said: “The order of the verses in their Surahs came about because this is how it was instituted by the Prophet (peace and blessings of Allah be upon him) and there is no dispute among the Muslims with regard to this matter.”
The texts regarding this matter include the following:

The report narrated by Ahmad, Abu Dawud, al-Tirmidhi, al-Nasai, Ibn Hibban and al-Hakim from Ibn `Abbas, who said: “I said to ‘Uthman, ‘What made you put al-Anfal which is one of the Mathani, next to Bara-ah [al-Tawbah], which is one of the Mi’in? Why did you not put the line Bismillah ir-Rahman ir-Rahim in between them when you put it at the beginning of the rest of al-Sab’ al-Tiwal [the long seven Surahs]?” ‘Uthman said: “The Messenger of Allah (peace and blessings of Allah be upon him) used to receive revelation of Surahs with many verses. When they were revealed, he would call his scribes and tell them, ‘Put these verses in the Surah where such-and-such is mentioned.’ Al-Anfal was one of the first Surahs to be revealed in Madinah, and Bara-ah (al-Tawbah) was one of the last parts of the Quran to be revealed. Its stories were similar to the stories mentioned in al-Anfal, so it was thought that it was part of it. The Messenger of Allah (peace and blessings of Allah be upon him) was taken [i.e., died] without explaining whether it was indeed part of it, so they were put next to one another, and the line Bismillahir Rahman ir’Rahim was not written between them, and it [al-Tawbah] was put among the Sab’ al-Tiwal [seven long Surahs].” (Al-Hakim said: its isnad is sahih, and al-Dhahabi agreed with him. Al-Mustadrak, 2/330)
Ahmad narrated in al-Musnad (4/218) with a hasan isnad from ‘Uthman ibn Abi’l-‘As that he said: “I was sitting with the Messenger of Allah (peace and blessings of Allah be upon him) when he fixed his gaze on something, then lowered his gaze until he was almost looking at the ground, then he gazed at something. He said, ‘Jibril (peace be upon him) came to me and told me to put this verse in this place in this Surah: “Verily, Allâh enjoins Al-‘Adl (i.e. justice and worshipping none but Allâh Alone Islâmic Monotheism) and Al-Ihsân [i.e. to be patient in performing your duties to Allâh, totally for Allâh’s sake and in accordance with the Sunnah (legal ways) of the Prophet in a perfect manner], and giving (help) to kith and kin (i.e. all that Allâh has ordered you to give them, e.g., wealth, visiting, looking after them, or any other kind of help), and forbids Al-Fahshâ (i.e. all evil deeds, e.g. illegal sexual acts, disobedience of parents, polytheism, to tell lies, to give false witness, to kill a life without right), and Al-Munkar (i.e. all that is prohibited by Islâmic law: polytheism of every kind, disbelief and every kind of evil deeds), and Al-Baghy (i.e. all kinds of oppression). He admonishes you, that you may take heed.” [al-Nahl 16:90 – interpretation of the meaning].’”
Al-Bukhari narrated in al-Sahih (no. 4536) that Ibn Abi Malikah said: “Ibn al-Zubayr said: ‘I said to ‘Uthman, “This verse that is in al-Baqarah (interpretation of the meaning), ‘And those of you who die and leave wives behind them … And those of you who die and leave behind wives should bequeath for their wives a year’s maintenance and residence without turning them out’ [al-Baqarah 2:234-240] has been abrogated by the other, so why are you writing it down?” He said, “Leave it as it is, my nephew, I will not change anything from its place.”’”
Muslim narrated (no. 1617) that ‘Umar said: “I did not ask the Prophet (peace and blessings of Allah be upon him) about anything more than I asked him about al-Kalalah [those who die leaving neither ascendants nor descendents as heirs], until he poked me in the chest with his finger and said, “Is not ayat al-sayf at the end of Surat al-Nisa enough for you?”
There are also the hadiths about the verses at the end of Surat al-Baqarah.
Muslim narrated (no. 809) from Abu al-Darda that whoever memorizes the first ten verses of Surat al-Kahf will be protected from the Dajjal; according to another version, whoever recites the last ten verses will be protected.

The Prophet’s recitation of various Surahs in the presence of his Companions indicates that the order of their verses is something that was instituted and commanded by him, and that the Companions did not put the verses in a different order from that in which the Prophet (peace and blessings of Allah be upon him) used to recite them. The reports to that effect reach the level of Mutawatir.
Al-Qadi Abu Bakr said in al-Intisar: 
“The order of the verses is something that is obligatory and binding. Jibril used to say (to the Prophet (peace and blessings of Allah be upon him)), ‘Put such and such an verse in such and such a place.’”
He also said:
 “What we think is that all of the Quran which Allah revealed and commanded that it should be written down, and which was not abrogated after being revealed, is that which is contained between the covers of the Mus-haf of ‘Uthman, and that there is nothing lacking or added to it. Its order is the same as that commanded by Allah, may He be exalted, and instituted by the Prophet (peace and blessings of Allah be upon him). Nobody has moved any verse backwards or forwards. The Ummah learned from the Prophet (peace and blessings of Allah be upon him) the order of the verses of each Surah properly, just as they learned from him the correct pronunciation and recitation.”
Al-Baghawi said in Sharh al-Sunnah: 
“The Messenger of Allah (peace and blessings of Allah be upon him) used to instruct his Companions and teach them what was revealed to him of the Quran in the order in which it appears in our Mus-hafs now, the order which Jibril taught him. When each verse was revealed, Jibril would tell him, this verse is to be written after such and such a verse in Surah such and such. Thus it is clear that the efforts of the Companions were limited solely to gathering the Quran in one place, not to arranging its order. The Quran is written on al-Lawh al-Mahfuz in this order, then Allah sent it down in its entirety to the first heaven, whence it was revealed in stages as it was needed, and the order in which it was revealed is not the order in which it is recited.”
Order of Surahs in the Quran
With regard to the order of Surahs, was this also instituted by the Prophet (peace and blessings of Allah be upon him), or was this the ijtihad of the Companions? With regard to this question, there were differences among the scholars. The majority of scholars, including Malik and al-Qadi Abu Bakr, according to one of his two opinions, favoured the second opinion.
Ibn Faris said: 
“There were two aspects to the compilation of the Quran. One was putting the Surahs in a certain order, such as putting al-Sab’ al-Tiwal (the seven long Surahs) first, and following them with the Mi’in, which is what the Companions did. The second aspect is the compilation of the verses within the Surahs. This was instituted by the Prophet (peace and blessings of Allah be upon him), who recited them as he was told to by Jibreel, on the command of his Lord. On this basis, the Salaf differed as to the order in which the Surahs should appear. Some of them put them in order of revelation, which was the mus-haf of ‘Ali, which started with Iqra (al-‘Alaq), then al-Muddaththir, then Nun (al-Qalam), then al-Muzzammil, and so on. The Mus-haf of Ibn Mas’ud started with al-Baqarah, then al-Nisa, then Al ‘Imran, with other major differences. The Mus-haf of Ubayy was similar.”
Al-Karmani said in al-Burhan: the order of the Surahs as it is now is the same as that in al-Lawh al-Mahfuz which is with Allah. This is the order in which the Prophet (peace and blessings of Allah be upon him) used to review the Surahs of the Quran he had received so far with Jibril each year. In the year in which he died, he reviewed it with him twice, and the last verses to be revealed were (interpretation of the meaning): ‘And be afraid of the Day when you shall be brought back to Allah…’ [al-Baqarah 2:281]. Jibril commanded the Prophet (peace and blessings of Allah be upon him) to put this between the verses dealing with riba and those dealing with loans.”
Al-Zarkashi said in al-Burhan: “The difference between the two views is one of wording. Those who support the second view say that it was taught to them so that they could learn the reasons of Revelation and the sequence of the words. Hence Malik said that they compiled the Quran according to what they had heard from the Prophet (peace and blessings of Allah be upon him), as well as saying that the order of the Surahs was the product of their ijtihad. He said that the dispute boils down to one question which is: was this a verbal instruction of the Prophet (peace and blessings of Allah be upon him), or were they merely referring to what he used to do, which would give them room for ijtihad.”
Al-Bayhaqi said in al-Madkhil: “At the time of the Prophet (peace and blessings of Allah be upon him), the Surahs and verses of the Quran were in this order, except for al-Anfal and Bara-ah (al-Tawbah), as we see from the hadith of ‘Uthman referred to above.”
Ibn ‘Atiyah said: “The order of many of the Surahs was known during the lifetime of the Prophet (peace and blessings of Allah be upon him), such as the seven long Surahs, those that begin with Ha’-Mim, and al-Mufassal. With regard to the others, it may be the case that it was left for the Ummah to determine their order after he died.”
Abu Ja’far said: “The reports support most of what Ibn ‘Atiyah said, and a little is left which is open to debate, such as the hadith, ‘Recite the two shining ones, al-Baqarah and Al ‘Imran.’” (Reported by Muslim, no. 804)
Al-Bukhari narrated (no. 4739) that ‘Abd-Allah ibn Mas’ud (may Allah be pleased with him) said: “Bani Israil (al-Isra), al-Kahf, Maryam, Ta-Ha and al-Anbiya, these are among the earliest and most beautiful Surahs and they are my treasures,” (meaning, these are among the earliest I recited).
Abu Ja’far al-Nahhas al-Mukhtar said that the Surahs were compiled in this order from the Messenger of Allah (peace and blessings of Allah be upon him) because of the hadith of Wathilah, “In place of the Tawrat I was given the seven long Surahs.” He said: “This hadith indicates that the order of the Quran was taken from the Prophet (peace and blessings of Allah be upon him).
Ibn Hajar said, “the fact that the Surahs, or most of them, are in a given order does not contradict the idea that this was instituted by the Prophet (peace and blessings of Allah be upon him). What proves that this was instituted by the Prophet (peace and blessings of Allah be upon him) is the report narrated by Ahmad and Abu Dawud from Aws ibn Hudhayfah, who said: ‘We asked the Companions of the Messenger of Allah (peace and blessings of Allah be upon him) in the morning, “How did you write down the Quran in chapters?” They said, “We wrote it down in chapters of six Surahs, five Surahs, seven Surahs, nine Surahs, eleven Surahs and thirteen Surahs, and the chapter of al-Mufassal, from Qaf until the end. This indicates that the order of the Surahs – as it appears in the Mus-haf now – existed at the time of the Messenger of Allah, or it is possible that what was known in order at that time was Hizb al-Mufassal specifically, which is not the case with the rest.”
(See al-Itqan fi ‘Ulum al-Quran by al-Suyuti, 1/62-65)
And Allah knows best.