Wisdom behind Fajr prayer being two rak‘ahs
Question

                Why is Fajr prayer two rak‘ahs, fewer than the number of rak‘ahs in other prayers?
Praise be to Allah.
    
    Firstly: 

    
    Allah has enjoined upon His slaves five prayers to be offered each day and 
    night, and He has prescribed the number of rak‘ahs in each prayer according 
    to His wisdom and knowledge. 

    
    So we have to accept, submit and obey, and to say as the Messenger and the 
    believers said: “We hear, and we obey” 
    [al-Baqarah 2:285]. For more information, please see the answer to 
    question no. 65877 

    
    Secondly: 

    
    Al-Bukhaari (1090) and Muslim (685) narrated from ‘Aa’ishah, the wife of the 
    Prophet (blessings and peace of Allah be upon him), that she said: Prayer 
    was enjoined with two rak‘ahs in each prayer, whether one was staying at 
    home or travelling. Then the prayer when travelling was confirmed (as being 
    two rak‘ahs) and the prayer when not travelling was increased. 

    
    Ibn Rajab (may Allah have mercy on him) said: When Allah, may He be exalted, 
    enjoined upon His Messenger the five prayers each day and night, then he 
    came back down to earth and Jibreel (peace be upon him) led him in prayer at 
    the Ka‘bah, his prayer at that time was only two rak‘ahs in each prayer, 
    whether one was staying at home or travelling. Then the prayer when 
    travelling was confirmed as it was, and two rak‘ahs were added to the 
    prayers when staying at home. What is meant here is the four-rak‘ah prayers 
    only.

    
    End quote from Fath al-Baari, by Ibn Rajab, 2/327 

    
    Ibn Khuzaymah (305) and Ibn Hibbaan (2738) narrated that ‘Aa’ishah said: The 
    prayers, whether travelling or staying at home were enjoined with two 
    rak‘ahs each, then when the Messenger of Allah (blessings and peace of Allah 
    be upon him) settled in Madinah, two rak‘ahs were added to the prayers when 
    staying at home, but Fajr prayer was left as it was, because of the length 
    of the recitation therein, and Maghrib prayer (was also left as it was), 
    because it is the Witr of the day.

    
    Classed as hasan by Shaykh Shu ‘ayb al-Arna’oot in his commentary on 
    Saheeh Ibn Hibbaan. 

    
    According to a report narrated by Ahmad (26338), the first thing to be 
    enjoined upon the Messenger of Allah (blessings and peace of Allah be upon 
    him) was prayer with two rak‘ahs each, except Maghrib, which was three 
    rak‘ahs. Then Allah completed Zuhr, ‘Asr and ‘Isha’, making them four 
    rak‘ahs when staying at home, and confirming the original obligation (of two 
    rak‘ahs) when travelling.

    
    Classed as hasan by the commentators on al-Musnad. 

    
    This hadith explains that when the prayer was first enjoined, it was two 
    rak‘ahs in each prayer, then that was confirmed in the case of travel, but 
    it was increased when staying at home. So Zuhr, ‘Asr and ‘Isha’ were 
    increased by two rak‘ahs. Fajr was confirmed as two rak‘ahs so that one 
    could recite at length in it. Hence the Prophet (blessings and peace of 
    Allah be upon him) used to recite at greater length in it than in the other 
    prayers. Sometimes he would recite Soorat as-Saaffaat in it when leading 
    them in prayer, as was narrated by Ahmad (4989) and classed as hasan by the 
    commentators on al-Musnad. Sometimes he recited Soorat ar-Room, as 
    was narrated also by Ahmad (15873) and classed as hasan by the commentators 
    on al-Musnad. Sometimes he recited between sixty and one hundred 
    verses, as is agreed upon (by the hadeeth scholars). So as to allow lengthy 
    recitation in Fajr prayer, it was confirmed as two rak‘ahs, as it was 
    enjoined in the beginning, not because two rak‘ahs were dropped from it. 

    
    Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said: 

    
    When Allah, may He be exalted, enjoined the five daily prayers in Makkah, he 
    enjoined them with two rak‘ahs each. Then that was confirmed when 
    travelling, and more was added to the prayers when staying at home, as is 
    proven in as-Saheeh from ‘Aa’ishah (may Allah be pleased with her), 
    who said: When he (the Prophet – blessings and peace of Allah be upon him) 
    migrated to Madinah, (two rak‘ahs) were added to the prayers when staying at 
    home (i.e., not travelling); Maghrib prayer was made three rak‘ahs, because 
    it is the Witr of the day, and Fajr prayer was confirmed with two rak‘ahs, 
    so as to allow for lengthy recitation therein, hence there was no need to 
    increase the number of rak‘ahs.

    
    End quote from Majmoo‘ al-Fataawa, 23/114. 

    
    A similar case is the shortening of Jumu‘ah prayer to two rak‘ahs because of 
    the khutbah. Ibn al-A‘raabi narrated in his Mu‘jam (1447) that 
    ‘Aa’ishah the Mother of the Believers (may Allah be pleased with her) said: 
    Allah, may He be exalted, enjoined the prayer upon His Prophet (blessings 
    and peace of Allah be upon him) in Makkah, with two rak‘ahs in each prayer, 
    except Maghrib, which is the Witr of the day. When he migrated to Madinah 
    and settled there, two rak‘ahs were added to each two rak‘ahs, except 
    Maghrib prayer, for it is the Witr of the day; Fajr prayer, so as to allow 
    for lengthy recitation therein; and Jumu‘ah prayer, in which the prayer is 
    two rak‘ahs because of the khutbah. 

    
    Ibn Qudaamah (may Allah have mercy on him) said: 

    
    It was narrated from ‘Umar (may Allah be pleased with him) that he said: The 
    prayer was made shorter because of the khutbah. The view of ‘Aa’ishah is 
    similar to that. Sa‘eed ibn Jubayr said: Jumu‘ah was four (rak‘ahs), then 
    the khutbah took the place of two rak‘ahs. End quote from al-Mughni, 
    2/224 

    
    And Allah knows best.