California also has a large proportion of East Asians, where mask-wearing is common, especially when you might have a cold, so that you don't spread it to the next person. Epidemiology is culture-bound. By now everyone knows that the incubation period for coronaplague is long and that pre-symptomatic people are a significant source of spread. It's not a long leap for an East Asian person to think that they might carry the disease without knowing and want to minimize spreading it.