Wherever this virus has gone unchecked the hospitals fill up quickly. I'm not aware of any run on hospital space in December of last year in California. The theory being that California had an early go at the virus and is why they didn't have high numbers like NY over the last couple of months. My point being, if the virus (at least the strain that is ravaging NY) had arrived in California back in December there would be evidence of it by the hospitals being overrun along with a higher number of deaths than usual.
I think there are other factors. I do not think the California spike (if there was one, hospitalized or not) was as high. California is car culture, not subway culture. individual space reigns in Cali, Shared space in NYC: Elevators, subways, transit buses and trains, taxicabs (shared space, just serially), heavy sidewalks, crowds. stairwells.
I think the opportunity for transmission in NYC is multiples what it is in California.
3700 deaths were summarily added to the death toll in NYC, with no evidence of the virus being involved. ("presumed")
Other environmental factors may apply.
We know how many dead there are in NYC (well sorta, the number is inflated), but not how many got well, were asymptomatic, or had mild cases.