Although it is kind of a riff on a Colin Cowherd theory so I have to give him credit.
Basically, the Pac 12 just cares less than any other P5 conference.

Every other conference is made up of powers in regions that CARE MORE about college football or basketball; whether you are talking about Football in the south or Texas/Oklahoma, or basketball in Carolina and Virginia, those fan bases are more loyal and obssessed.

In the Pac 12, the flagship program is USC, but USC is still second in the city to the Lakers, and the Rams and Dodgers have both been on an upswing for the last few years. Combine this with the fact that LA is a town full of transplants anyway, and they have the movie industry and good weather to distract them. UCLA basketball faces all of these same issues.

In the Bay Area, neither Cal nor Stanford have a history of success, it is a huge pro sports town currently dominated by the Warriors. Plus both of those fan bases pride themselves on being nerds instead of jocks so even when Stanford football is in the top 10 they don't fill the stadium.

Denver is a professional sports town, and the Buffs have been terrible for a decade in football and historically terrible in basketball so fan interest continues to erode.

Washington seems to be the only program who has it together, perhaps due to good coaching. But even Seattle can be a pro sports market when the Seahawks are good.

You all know how Utah works, sometimes the Jazz dominate the market, and BYU has some market share.

The net result is that there are no places in the Pac 12 that mirror College Station, Columbus, Tuscaloosa, Gainesville, Norman, etc., where you combine historical success and a large, passionate fanbase.