I'm not sure what you don't get. RP supporters have no problem admitting and pointing out that he's the black sheep of the Republican Party, but is it really that tough to figure out why? He doesn't run on the GoP's platform, and then he claims that he runs on what the "true" platform of the GoP is, even though he can't factually point to a time when the majority of the GoP shared in his ideology and backed it up with their actions.
People (like most of my family) who've been voting GoP most of their lives see him as a pretender. And, to be brutally honest, that's what he is. In fact, most RP supporters I've met have been people who are anything but actual Republicans. This includes people I know who are left of most democrats, libertarians(not REPUBLICANS for a reason!), and teenagers getting out of their Michael Moore phase. But very few people who actually are card-carrying GoP members.