Because, in general disabled people are still able to drive - there are some quite ingenious aids for that. In the context of wheelchair users, that includes a roof-top storage "bin" that will deploy, winch your folded chair up when you've got into the driver's seat, and stow it - dispensing it back again at the other end.
Contrast that with (some) busses which might as well not exist as far as a wheelchair use is concerned as there's no way for then to get on and off.