There are several reasons for that. One of them is that MGs have a very limited use case. I've worked with them extensively, and I can't think of a hole they would fill in my shooting life; I'm not suppressing enemy fire nor am I maintaining an FPL, which are the roles MGs fill better than anything else they do.
Of course I support a man's right to buy an MG if he wants to, but I'd never bother. To that end, while I'd strongly object to an MG ban on constitutional grounds, I can't say it would affect me in the slightest. The ban would be less likely to mobilize or [further] radicalize me, to be truthful.
As you point out, you need political will to start that ball rolling. If I were a politician, owing favors to many constituents, the MG lobby would probably be pretty far down on my list. There are infinite things I'd need my political capital for, and most are more important than MGs.
Just calling it like I see it.