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