Large, wealthy, lots of resources, politics somewhat different from national average, history of independence and strong state identity.

Basically the same reasons as Texas. There are actually some pretty significant shared cultural elements though people get angry if you point this out.

To add to the other answer, good agriculture as well. They can handle feeding themselves just fine. But they’d have to take AZ with them as well or else be able to negotiate like hell else SoCal won’t get any more water from the Colorado. AZ would not join CA willingly.