The answer, as always, is money.
Former Pac-12 commissioner Larry Scott bet on creating the Pac-12 cable network. That failed, which left the Pac-12 in a bad position when it came to TV rights. In addition, with the existing TV rights expiring after this season, there was no penalty to leaving the conference.
Seeing the writing on the wall, USC and UCLA defected to the Big 10. Since they represent the second largest media market in the US, they got a sweetheart deal–a full share of Big 10 TV money, or about $75 million per year. That left the Pac-12 (now with 10 schools) scrambling to find a TV home.
Colorado, smelling what was up, defected to the Big 12, causing even more turmoil.
Eventually, the best offer the Pac-12 got was about $20 million per school per year from Apple. That prompted Oregon and Washington to jump to the Big 10, but because they were in a position of need, they are only getting a partial share of TV money until the next deal. Still, that $30 million per year partial share was better than what the Pac-12 was offering.
Having lost five members, the remaining Pac-12 schools began looking for lifeboats. Utah, Arizona, and Arizona State defected to the Big 12 (which started this whole game of musical chairs years ago, when Texas and Oklahoma defected to the SEC), and will get a full $32 million share of Big-12 TV money starting in 2025.
That leaves Stanford, Cal, Oregon State, and Washington State. Stanford and Cal are desirable, and could end up in another conference, but will likely make less money than before, so the next effect of conference realignment might mean those schools having to cut some of their non-revenue sports to balance the books.
Oregon State and Washington State are screwed. They had it good in the Pac-12, and have no leverage with any other conference since they are their former conference’s two weakest media markets.
None of this is good for the Pac-12, college athletics, or the schools involved, but USC and UCLA look the smartest (if the most ruthless) for getting out when they had the most leverage.