Because that state still has 1/3 of the population that has ALWAYS been hardcore right-wingers.
Did you honestly think that California was some monolithic super-liberal state? Have you ever been to its capital city of Sacramento?
Of course there were PLENTY of Californians who weren't following social distancing rules to the best of their ability, Trump absolutely conned them into being "defiant".
Secondly, interstate travel is still allowed, which means people from Red states would be entering California.
And lastly, here's the real answe