Crew domiciles are not always major partner hubs and vice versa...
Maintenance bases play into this also.
Usually a large major partner hub is also a regional crew domicile, but not always.
Regionals often will have crew domiciles at Mx bases since they will flow a lot of airplanes through for periodic checks. This is the case for FAT, FAT, and PSP. Also it's cheaper to have crews based...
1) Outside CA
2) Rural CA
3) SFO / LAX as a last resort => very high employer taxes.
Additionally some majors will have mini-hubs served mostly or entirely by regionals...FAT used to be one of those, maybe still is somewhat.
An airline can always flow crews in from other locations if that makes sense...they might end up with a bunch of crews overnighting in hotels but sometimes that's easier than opening a domicile. SFO used to have a LOT of overnight crews, and only a tiny handful of reserves actually domiciled there.