It's actually down to two reasons. Firstly California is probably the first part of North America to see continuous permanent settlement by humans, possibly including Non-Sapien genus Homo but that's still up for debate. Mostly because everything further north was a frozen shithole on par with Svalbard.

Secondly the various tribes of the West and especially in California were able to actually recover somewhat from the wave of plague the Spannish unleashed. Mostly because between the Sonora, Mojave, and Great Basin it meant that it took centuries for Europeans to get into California. Though the Russians may have been fucking around Oregon if a rather well traveled Native American from out east is to be believed. Sorry I can't remember the dudes name or his tribe I think he was from Arkansas.