The problem (in my opinion) is that it isn't possible to implement it in a way that would do it justice.
Let's look at just one disability: blindness.
Some blind people confine themselves to their homes all day in fear of the outside world. Others live almost completely normal lives. In which way, then, will the blind be represented? What type of limitations would we give these sims to separate them from normal sims and would those limitations be accurate for all or even most blind people?
This just shows that if some sort of disability system were added, it cannot be cookie cutter. It can't be simply "pick a disability and your sim can't do blablabla as well as other sims." It would need to be a massive system that provides more customization on the player's end than anything we've seen added to the game thus far. And for me, it just isn't worth that amount of resources.
