As fun as it is to call the other side dumb, I don't think that's it. He appeals to angry working class white people. The Republicans have been misdirecting their anger at minorities for a long time to get their support, but they're more sophisticated now. They're not going to go to the left, they'll be expected to do too much self-flagellating, and they're not secure enough to shrug that off. Trump takes some traditional left-populist positions, opposition to trade deals being the big one, and he's not going to make them feel bad for America's history of racism and misogyny because he's still racist and misogynist. He's manipulating people just as transparently, he just has a more modern strategy for doing it.
Perhaps uneducated? Or willingly ignorant, take your pick.
Angry, frustrated, afraid, those work better. People voting for Trump are not dumb; I'm surrounded by them and don't fucking dare put up a Sanders decal unless I want rocks thrown through my windows. People outside of the big tech cities are watching amazing things happen somewhere else as their way of life is slipping away... and nobody seems to care or even give a shit as long as the Internet is working and the pizza is being delivered for the sports stuff this weekend. Trump himself will be a joke and I am not afraid of him. I'm scared and concerned about what friends of his will be running the FCC, SEC, DEA, FDA, EPA etc. That is where the real fear should be. Of all the candidates, the only one that I think gives a damn about anyone who really needs to be told "hey, it sucks now but I'm working on it, I got your back" is Sanders. And the neat thing to watch is that the more some of these diehards are hearing about him because they hate the Clintons, the more they are telling me that he makes sense and seems genuine.