California law limits how high the insurance companies can charge for premiums. Did that law or those limits exist when they started offering coverage in the area?

Maybe they didn't, and then the law or limits were imposed at a time when the insurance companies needed to increase the premiums to match the new risk. But if the law prevents them, then they have no other choice but to pull out. Why would they as a business stay if the risk is to great for the premiums they are allowed to charge? They certainly are not obligated to stay.