It's not limited to California. Try getting a reservation in south Florida on short notice. We were reserving 2-3 weeks ahead on last winter's trip and that stopped south of Ft Myers. Oh, if we wanted to pay $125 a night there were sites available, but anything public, COE or reasonably priced was booked a year out. Some beach sites were so tight that you couldn't put your slides out. At that point we turned back to the north.
The 'secret' down there is to make your reservations for next year when you leave this year. However, there are some public parks that don't allow this, and you need to play the Southwest Airlines reservation game.
So I guess CA has this problem because the regulatory process to open a $125 a night beach front campground is too ominous. Life in a blue state...