All the other sites have a mobile version, but Area 51 doesn't. Why not? Can it be added?
Area 51 is actually a separate codebase from the rest of the Stack Exchange sites.
At this point it is sort of on its last legs as we're working towards "Area 52", which is probably/hopefully not going to be actually called "Area 52".
We currently don't have any plans to put a lot of development effort into Area 51 as a result, and that includes creating a mobile interface for it. No promises at this point, but fingers crossed for a mobile-friendly Area 52. :)