Because I'm the token Socialist on the forum, I know there are quite a few armed Leftists.
Most Democrats, even if they realize that the NRA is largely responsible for protecting their RKBA, incorrectly see it as exclusively supporting Republicans and they don't wish to contribute to that.
Also, the recent upsurge in hoplophobic propaganda has hurt the public image of the NRA so much that some people are afraid to be associated with it.
My wife and I, however, are standing firm with the NRA.