I am going to probably rile some feathers here, but the primary cause for that conference's demise was their fans.

Here me out

When Pete Carroll was coaching USC and they were near the top 10 every year, they were drawing 90K fans a game to the LA Coliseum. Last year, even with Lincoln Riley and a Heisman QB, they drew only 65K a game, a 32% decline in less than 20 years

UCLA - When Jim Mora was coach from 2012-2017 they drew 65K a game, last year it was only 41.5K, a drop of 36% in just 6 years

Stanford - used to play in a stadium that held 89K but in 2005 downsized to just 50K, and last year got less than 30K to show up. They averaged 43K just 3 years ago before covid. That's a 30% drop in 3 years

Cal - in the 5 years from 2015-2020 Cal ranked dead last in the conference is percent of capacity and 9th in attendance until last year drawing less than 40K a game, when they were drawing close to 60K a game in the 2003-2006 era. thats a drop of 25% in 15 years

Oreg St - was drawing close to 46K a game back in 2010, last year drew only 31K a drop of 33% in 12 years.

and on and on. Viewership was down for nearly every program as well.

Essentially their respective fan bases abandoned them. Basically only Oregon and Washington held firm or grew over the last 10 years.

How much of this is lack of interest, costs, bad marketing, covid impacts on field performance, etc isn't clear, but while schools like Ohio State and others remained high in attendance and viewership, almost all the Pac 12 schools saw declines with many being significant. 

In the end their fan bases shrank and lost interest and hence their top schools became prime targets for picking.