Based on my research, presumably there does seem to be the necessity of much/diverse illustrations (from different aspects) in order to present a complete/adequate response, but I strive to answer (at least by having a short explaining regarding an aspect of …, anyhow. From the viewpoint of Shia scholars (except a minor group of them), the sources of Fiqh is four things: The Qur’an, Sunnah, Ejma’(consensus) and Aql.
Of note, as surveying from an aspect (based on my research + my assumption):
Whereas humans need to have diverse permissible enjoyments such as travel (to be reset/fresh of some/much tiredness and …), then Allah gives a change/discount for travelers to have an easier daily religious responsibility (besides, the travel itself have its tiredness, too, due to …), hence Allah tells them so to be more comfortable.
But, this order/opportunity of Allah is related to traveler, not for the ones who are going to trip for haram acts, because in that case they (logically) won’t be deserved of such opportunity/discount of Allah…
On the other hand, the one whose job is traveling, then in essence he/she is not travelling (of enjoyment…) but actually s/he is doing her/his profession by that way hence its job won’t be like travelers…, so he/she won’t be deserved of that discount based on that formula of Allah. Even, logically we can say so that: A common person who intends to take trip to have an enjoyment/rest (after a long period of time of work which has made him tired) would be deserved of discount of that, but the one whose job is travelling … consequently it is not fair that both of them have the same discount of that!
But, interestingly, (AFAIK), e.g. the one whose job is travelling, but someday he/she intends to travel to a city (but not as his job, whereas with the intention of only traveling like other common travelers, subsequently s/he will have the condition of traveler and would have such profits of travelers.
Source and complete information: