There's a part of me that really likes the paired in-state rivalries in the Pac-12. UO-OSU, UW-WSU, USC-UCLA, Cal-Furd, UA-ASU. So pairing CU with CSU or AFA while pairing Utah with BYU or USU has a certain appeal.
More over, I wish that none of the states divided affiliations with power conference teams in different conferences. Iowa and ISU should be together. UGA-GA Tech, Clemson-SCar, Penn State-Pitt, UT-TTU-TCU-aTm-BU, UF-FSU-Miami, Kentucky-Louisville. Regionality and local rivalries are a big part of what make college football different and special.
But unless the television networks take over the alignments, this isn't how the modern landscape works. When the conferences are competing with each other for media markets instead of it being a national league with a focus on maximizing fan interest within each market, it changes the dynamics considerably. Texas Tech is more valuable to the Pac-12 than it is to the Big 12. Virginia Tech would be more valuable to the B1G than to the ACC. A national body would be more concerned with filling stadiums and drawing the biggest possible number from each game on the schedule, not with gaining additional distribution for a conference.
So, in the world that is, CSU makes no sense for the Pac-12. It would cost other members millions while diluting an already mediocre state for recruiting. Even a New Mexico, Boise State, UNLV and SDSU - which bring new states or major western metros and drivable games for conference members that would do well for fan base building - they don't even move the needle since the conference already has PACN distribution in those places. Under the current format, it's pretty much Pac-12 expansion into Texas or standing pat.