It's all of the above.
Does someone know why people using wheelchairs have to bypass the first part of the Haunted Mansion? It's been awhile, so I don't remember if there are stairs or some other problem, and DD was curious about that.
Thanks!
Mary
No stairs, but there are other problems.
I'm sure someone else has a good answer but I think it has to do with a safety issue concerning the wheelchair in a darkened area. Karen
The dark area is one concern.
The stretching room itself is dark, and the area after that which leads to the boarding area is very dark.
I thought it was just that the turnstiles are too narrow for a wheelchair?
The turnstiles coming into the building are too narrow for a wheelchair. Probably one reason they have not changed those is that they really don't want people entering into the buidling there with wheelchairs anyway.
I thought it was because it was a continuous ride, so you could be loaded at the exit, rather than the entrance and have more time to load/transfer if necessary.

Does someone have the real answer, I'm intrigued now!
For the continuous loading rides, the CM at the exit has more control over the speed. When you enter from the 'usual' boarding area, the CM doing boarding doesn't see you until you are right at the boarding area, ready to get on.
You have more space and time to get on if you are boarded at the exit. The CM can also 'hold' you there until a particular car comes around. In the case of Haunted Mansion, there is not a wheelchair car, but there is a car that has a wider open door, which makes transfers easier.
Queue area is very narrow and a wheelhcair would not fit unless they have changed it recently.
The queue to board starts off wide, but it gets narrower and narrower as you get closer to baording. The function of the narrowing is to 'funnel' the guests down into a single file line for boarding.
As Talking Hands said, the dark boarding area, after exiting the stretching room, is way too narrow to allow for the turning radius of even a manual wheelchair. If the turnstyles that count guests were the only problem they could have easily (and pretty inexpensively) been replaced with the laser/electric eye people counters they use at EPCOT shows.
One more reason I can think of wasn't mentioned yet.
The boarding area and the exit are not in the same place. Close, but not real close.
If you boarded with a wheelchair at the 'regular' boarding area, a CM would have to bring the wheelchair from the boarding area to the exit to be waiting for you when your ride is over. There is very little room at the boarding area for wheelchairs to be parked while waiting to be taken to the exit.