Because it wasn't like this until the NRA ramped up their identification with the right. Again, as recently as 2010 there was a large minority of openly pro-gun people on the left.
To back that up, I know a great many left-leaning gun owners in LA that would never join the NRA as a matter of principle (due to its extreme positions), including members of law enforcement, retired military personnel, and regular joes who just like guns and things that go boom.
