The Pac 10 is the gemini conference, made up of twin universities in each state/region of the west, and it has remained unchanged for 30 years. There's a lot of people who would like to see the conference add two more members to make 12, but that's just not going to happen - ten is the number of balance for this conference.
One of the main reasons the Pac 10 won't add two more schools is that the presidents of these universities don't feel that any other schools in the west measure up to their institutions' high "Research I" standards. Whether or not that's true doesn't matter - it's what they believe. And this group of university presidents is about as tightly knit as you're going to find in the NCAA. They're probably more unified than any other group of administrators in their conference-wide decision making, and they're unified against not expanding further.