I think it's a combination of the personal, emotional attachment many developed after 9/11, the pro-abortion wing of the party trying to assert itself and a lot of people who fundamentally agree with the 2nd Amendment but have don't any real personal stake in it (or so they think, because they don't own firearms).
I think among the pro-aborts, you might find some closet Hillary supporters. Otherwise, I just don't think people are thinking this through.
Fortunately, there's still time.