The Pac12 dominates the west coast major metro markets with presence in Denver, Seattle, Salt Lake, Phoenix, Portland, SF, and LA.
THAT IS WORTH A LOT OF $$$$
Not to mention is geographically convenient. Remember the P12 faces the same issues as Gonzaga when looking to move conferences. If you're not interested in the Big West, WCC, WAC or Mtn West... it's just cost prohibitive to join a central or eastern conference.