It has to do with the way they build them. Virtually all other trailers are built from the chassis up. Frame, floor, interior stuff, walls, and finally roof. Airstreams are built from the outside in. And it makes for a fascinating factory tour. The chassis and floor are built conventionally. The shell is built in a jig starting with the outside skin. Add windows, door, inside ribs, and sealant and then they pick it up and set it down on the chassis. From that point everything else inside comes in through the front door. That is why the entry is wider than other RV's. If you look on shorter Airstreams forward of the door
there isn't enough room to mount an accessible door holder before the curve starts.

Charlie