I'm sure part of it was academic/cultural elitism, but I think a big part of this was a not-so-subtle warning from the Big 10.

The Big 10 wanted the strongest alliance possible to counteract the SEC and that meant keeping the Big 12 viable. Best guess is that the B1G told the PAC that if the PAC expands that the B1G would have to seriously look at expanding as well and would seriously consider ALL candidates, even those more geographically removed. Adding BYU, Oklahoma St, Houston, TCU and losing USC, Oregon, Washington, Stanford would be a huge net negative for the PAC12 and not something they wanted to risk