There was a thread about this on the disability board recently (I wrote it because my DD, who uses a chair, wanted to know why she couldn't see the stretching room). Apparently, there are several reasons. One has to do with how dark it is, there are tight turns (in the dark) after the stretch room, and the area where you board is not the same as where you exit, so someone would have to transport the wheelchairs over. There may have been a couple more too, but those are the big reasons. I've never been to
Disneyland, but I was told the set up there was different, similar to what the other posters said.
Mary
