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

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.