The Wikipedia page is interesting. The UES is the only part of Manhattan where registered Republicans make up more than 20% of the voters, and in one small part of the neighborhood there are actually as many registered Republicans as Democrats (the only part of Manhattan where that is true). It's no longer WASP Central. By one estimate, a third of residents are Jewish. But the district looks a lot like America used to. Wikipedia says the neighborhood is 88% White. That can't actually be true, can it?
The problem is the single women. The neighborhood hasn’t changed that much ethnically. Run a lefty woman and you have good odds of beating the Pubbies. We used to call it The Girl Ghetto back when I lived there. It was considered a safe neighborhood and is fashionable still if you’re not a downtown type of person. Some of these women even become Republicans because it is more fun and less declasse, but then they vote “for the person, not the party” (translation: they spend more time having their hair done than reading the newspaper, so they don’t know what the real issues are).
Another factor is all the apartments that are still owned by the wealthy but the wealthy no longer live there officially. They are in a state with no state income tax for 183 days a year, and then they have to vote there, not in New York.