Zamfi nailed it - NYC is 10x the population, but Manhattan is not. If you've ever tried to get a cab in both NYC and SF, you know that difference in medallions is very real.
I think I maybe once saw a cab in Brooklyn, but it turns out it was just a yellow car driving like a maniac.
(99% of taxi trips in NYC start or end in Manhattan. There are no cabs in the outer boroughs and they will only take you there if you get in, close the door, and get out your phone to call 311 if they refuse to take you. I don't even try anymore.)
