[81alum]

This is the part that I found most informative:

What internal characteristics/weaknesses have made the Pac-12 vulnerable to collapsing under the shifting dynamics of college football?
A: It’s hard to pinpoint one thing, but I think as the Pac-12 struggled on the football field, the interest simultaneously waned in a few other areas.
First, fans weren’t showing up with the same fervor. Secondly, local kids may not have been exposed to football as much and didn’t play in the same numbers as compared to the rest of the country. That has an effect on the number of future recruits and high-level athletes who are playing the game, as well as the alumni and fan base who follow the teams.
Some of the best young athletes who grew up out west, and who often would have played for a Pac-12 school, sometimes looked elsewhere — including Alabama.

Interest in football among west coast people is simply falling.  It may be as simple as that.

[81alum]

This is the part that I found most informative:

What internal characteristics/weaknesses have made the Pac-12 vulnerable to collapsing under the shifting dynamics of college football?
A: It’s hard to pinpoint one thing, but I think as the Pac-12 struggled on the football field, the interest simultaneously waned in a few other areas.
First, fans weren’t showing up with the same fervor. Secondly, local kids may not have been exposed to football as much and didn’t play in the same numbers as compared to the rest of the country. That has an effect on the number of future recruits and high-level athletes who are playing the game, as well as the alumni and fan base who follow the teams.
Some of the best young athletes who grew up out west, and who often would have played for a Pac-12 school, sometimes looked elsewhere — including Alabama.

Interest in football among west coast people is simply falling.  It may be as simple as that.