Spill over from Manhattan. Everybody works in Manhattan, but it became too expensive to live there so everybody looked for the next cheapest thing. So people start to move to all the neighborhoods around downtown and midtown Manhattan: the neighborhoods in Brooklyn and Queens by the water and Harlem in Manhattan.

But soon, even these neighborhoods become too expensive, so people start moving into the next neighborhood over, and the next, and the next... So that is why it's so expensive.

And these people are living in nice renovated apartments surrounded by fancy restaurants and shops. The opposite of how those neighborhoods were when the mafia thrived. Instead of fixing the neighborhoods, Italians, Jews (non-hasidic) and others just left for the suburbs. Many people blame this on "white-flight." Which is why you see most of these populations in Staten Island, Long Island, Westchester, New Jersey, etc...