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One major factor is just how early California ordered residents to shelter-in-place (or "lockdown"). Several San Francisco Bay Area counties issued an order on March 17. Governor Newsom then issued a statewide order on March 19. On that day, California had 1,006 cases. Compare this to New York's 7,102 cases on March 20 when its residents were ordered to shelter-in-place, or Italy's 9,172 cases on March 9 when its national lockdown was instituted. So New York and Italy were already quite behind to start with.

California also has a few other things going for it, for example: CA's urban areas are not as dense as somewhere like NYC. Compared to Italy, the population is relatively young. Many people have professional jobs that can be done remotely (companies like Google and Facebook sent their employees home even before March 12). Public transit is very bad in CA, so almost everyone drives their own car.