Like the comment protist copied from Wikipedia, it’s generally considered to be a mix of coming out with the truth, and having skeletons in the closet. Back in the eighties, that was the explanation I heard most often when hanging out with gay folks.

Doesn’t necessarily mean it’s absolutely factual, I’m not aware of anyone that’s really dug into it with serious historical rigor. But it is at least the accepted explanation. And it makes sense, so I’ve never gone digging beyond talking to people alive and active during the early gay rights era of the seventies.