The answer to that goes back to what I mentioned earlier. It's because the builders have never actually spent time (generally speaking) in a wheelchair or in a state in which they need that stall. They have no clue what would actually be the most useful for someone.
I looked up the ADA rules, and there is nothing in there that requires it to be in the back.
That said, also see my earlier post about having to leave the door open more than once myself. I have to admit that, in those times, I was glad that the stall was as far away from the main door as possible.