I made a post on the old board talking about football in the PAC. I think part of it is purely geography. The B1G, B12, and SEC are highly clustered. The ACC is a bit of a far-ranging conference, but it's in an area where everyone (absent the North East) follows CFB.
The P12 is spread FAR and WIDE. You have geographic rivalries (USC/UCLA, UA/ASU, Stan/Cal, OU/OSU, UW/WSU), but basically outside of those, there really aren't a lot of "driving distance" games. A lot of B1G or SEC games are available within a 4-5 hour drive. If you live in the "center" of the P12, i.e. the Bay Area, there's nothing within 6 hours drive.
So you add all the other things, like beautiful weather into November, like strong pro sports pretty much everywhere except Oregon and Utah, like the fact that so many people are transplants, etc, and people just don't devote their Saturday to CFB the way they do in other parts of the country. They might watch their own team, but they don't sit and watch football from the early B1G kickoffs at 9 AM all the way to the late P12 games.
Oh, and I don't think "their own team" is even as big of a thing in CA. Stanford and USC are very small, private schools. UC schools in general are VERY difficult to get into, and there are a lot of them. So most people here in CA didn't attend those schools. They went to a different UC, or to a Cal State school, or Cal Poly, or private, etc. The ones who are still CFB fans despite not attending one of those are usually so because one of their parents went to one of those big four.
Culturally, most of the people I know here in SoCal that care about CFB aren't from here. USC gets a lot of bandwagon fans when they're good, but those are USC fans, not CFB fans.