I'm going to guess that part of the reason CA prices are so much higher than the rest of the country is taxes. I just checked and the gasoline tax in CA ($0.511), is almost double what it is in my state of MN ($0.285) per gallon.
Another reason is the emission standards required by the state of CA due to the high smog issues.
Another reason is there is a lack of refining infrastructure to meet the needs according to a KTLA news report.
It is my guess that with California's recent push to outlaw the sales of ICE vehicles within the next 12 years, it will only get worse.