Basically, he was an expendable media whore. And his absence makes the redistricting puzzle a little easier to deal with.
I mean, the best case in a special election is that a Dem wins and everyone says, “Well, of course a Democrat won—it’s a Democratic district in New York City.” It’s be like kicking an extra point—you get no credit for doing it. The worst case is the race actually gets close, because most people don’t seem to recognize how R-friendly the 9th can be, and it blows up into another big national story, with the Republicans crowing that it’s a referendum on Obama and Democratic misconduct.
Then the Dems would have to spend money. Even if they pulled out a win, it would still be a headache: They’d get no PR value out of it and they’d have a new incumbent battling a bunch of other NYC Dems on the redistricting jockeying.
But if it stays vacant, there’s no special election and it can easily be carved up next year in a way that fortifies the other Dem incumbents around there.