I think there are a couple of factors to be considered:
- distances between schools are much greater in the West than
in the East (not even counting the Alaska schools).
- fewer fans will make a long trip to a regional one week after the
league tournament, at a cost of $50-60 for tickets, when their
team may only play one game.
- finding a host school and organizing committee. If the regionals
are run at all like the finals, the NCAA requires a hefty guarantee.
--david