The question was "Why do only cater to the former?" not whether or not it makes sense to do so. In fact the very question strongly implies that it doesn't make sense.
Non disabled persons using older browsers are used to things "just working" for them, so they have a lot of energy to expend being pissed off when things don't work.
Disabled persons are confronted with lots of things not "just working" for them, so the energy of being pissed off gets more spread out.
In the general case, this causes a positive feedback loop where minor issues for people without major problems get fixed, while more significant issues for people with even worse problems don't get fixed.
