Why aren't travellers mentioned in the hadeeth, “Jumu’ah prayer is an obligatory duty for every Muslim except four”?
Question

                In a hadeeth of the Messenger (blessings and peace of Allah be upon him) it says: “Jumu’ah prayer is an obligatory duty for every Muslim except four.” Why is it that travellers not included among those for whom Jumu’ah prayer is waived in this hadeeth?.
Praise be to Allah.
      The hadeeth referred to in the question was narrated from 
      Taariq ibn Shihaab (may Allah be pleased with him), that the Prophet 
      (blessings and peace of Allah be upon him) said: “Jumu’ah prayer is an 
      obligatory duty for every Muslim, to be offered in congregation, except 
      four: a slave who is owned, or woman, or a child, or one who is sick.” 
      Narrated by Abu Dawood (1067). Al-Nawawi said in al-Majmoo’ 
      (4/483): Its isnaad is saheeh according to the conditions of the two 
      shaykhs (al-Bukhaari and Muslim). Ibn Rajab said in Fath al-Baari 
      (5/327): Its isnaad is saheeh. Ibn Katheer said in Irshaad al-Faqeeh 
      (1/190): Its isnaad is jayyid. It was classed as saheeh by al-Albaani in
      Saheeh al-Jaami’ (3111). 

    It was narrated from Ibn ‘Umar (may Allah be pleased with 
    him) that the Prophet (blessings and peace of Allah be upon him) said: “The 
    traveller is not obliged to pray Jumu’ah.” Al-Haafiz said in Buloogh al-Maraam: 
    Its isnaad is da’eef. 

    The exception of the traveller is also mentioned in a number 
    of hadeeths, including the hadeeth of Abu Hurayrah, according to which the 
    Prophet (blessings and peace of Allah be upon him), who said: “There are 
    five for whom Jumu’ah is not obligatory: the woman, the traveller, the 
    slave, the child and the people of the wilderness.” It says in Majma’ al-Zawaa’id: 
    It was narrated by al-Tabaraani in al-Awsat and its isnaad includes 
    Ibraaheem ibn Hammaad, who was classed as da’eef (weak) by al-Daaraqutni. 
    Shaykh al-Albaani (may Allah have mercy on him) said: (He is) da’eef jiddan 
    (very weak). 

    It is no secret that the shar’i rulings should be derived 
    from all of the hadeeths that were narrated concerning this specific 
    subject, and they should not be derived from only one hadeeth whilst 
    ignoring the rest of the hadeeths. 

    The Prophet (blessings and peace of Allah be upon him) 
    mentioned a number, as we see in the hadeeth being asked about here, and 
    mentioning a number does not necessarily mean that it is limited to this 
    number. The aim may have been to make it easier to remember this hadeeth for 
    those who heard it. A similar example is the words of the Prophet (blessings 
    and peace of Allah be upon him): “There are seven whom Allah will shade with 
    His shade on the Day when there will be no shade but His…” By compiling all 
    the hadeeths which speak of this great matter (“Allah will shade them with 
    His shade”) it becomes clear that there are more than 20 such categories, 
    not only seven. 

    Al-San’aani said in Subul al-Salaam: 

    We can see from the hadeeths that Jumu’ah is not obligatory 
    for six people: children, as there is consensus that Jumu’ah is not 
    obligatory for them; slaves, as there is consensus on this matter except for 
    Dawood; women, and there is consensus that it is not obligatory for them; 
    sick people; travellers, who are not obliged to attend it; and the sixth 
    category, which is the people of the wilderness. End quote. 

    And Allah knows best.