Angels (mostly) come from startups. Each really successful startup yields a new batch of them. So by default a city's status as startup hub, or not, is self-perpetuating.

New York may be somewhat of an exception. There may be enough investors in NYC who are comfortable with risk that they're willing to invest in startups even though they don't come from the startup world themselves.

New York is also an attractive enough city that there may be a decent amount of successful startup founders moving there out of preference for the city.