A pro-choice gun nut won’t vote Democrat because he cares more about guns than the right to choose. But when you put it as a ballot initiative he doesn’t have to weigh it against anything else.
The thing about single-issue voters is that they can swing elections even though they’re a minority. The GOP has survived for decades by embracing the pro-life and pro-gun voters. These are massive groups with voters on one side who will base the entirety of their vote on the issue, while the other side of the debate doesn’t.