1)If the Pac-10 wanted to expand, it would have done so long ago.
It hasn't done so because it is primarily an academic and non-football athletic conference. It bills itself the "Conference of Champions" and for good reason. The bulk of its non-football athletic teams are competing for athletic championships. The three all-time leaders in NCAA overall team titles are UCLA, USC and Stanford and they're ahead of anyone else by a mile.
It also has three academic powerhouses in Stanford, California and UCLA and two near-powerhouses in USC and Washington. Any new faces to the mix will have to look a lot more like these guys than say, the University of Arizona.
2)Any of several possible pairs of teams could have entered long ago in maintaining the Pac-10's unique regional pairings (USC and UCLA, Stanford and Cal, Washington and Washington State, etc.). Think Utah and BYU, Utah and Utah State, BYU and Utah State, San Jose State and Fresno State etc. etc. etc.
It's never happened.
3)The Pac-10 is stubborn. It is the only major conference not to speak up with plans for its own television network when a flurry of those announcements went out last week. It has held to its ten-team unity and I doubt it sees any compelling reason not to. Remember this is the conference that had to be financially blackmailed, kicking-and-screaming, to join the BCS, thus sacrificing its and the Big Ten's exclusive ties to America's greatest bowl tradition (the Rose Bowl) and the financial windfall it provided for the conference.
Maybe someday the Pac-10 will cave to the almighty dollar (many of us hope that day never comes) and add two members, do the whole two-division and sell its soul. But that day is not today, nor is there any scent in the air to suggest otherwise.
Entering Big 12, SEC and ACC territory
Getting back to the original argument: Colorado is not a natural fit for the Pac-10. Nor is Utah, or BYU or Colorado State or any similar teams in the non-Pacific American west. I don't consider any of them to have prestigious academic credentials (strike one) or overall athletic depth beyond football (big strike two).
I don't see the Pac-10 seeking those opportunities.