Alt conservatives keep talking about Ben Carson's popularity being due to white guilt. As Vox says, he's the get-out-of-racism-free-card candidate in their eyes, the guy whose main appeal is that he can be a black man conservatives support, shield them from charges of being evil bigots.
Yep. There's probably some truth to that. But I think the reasons to support him go beyond that for a long-game conservative.
See, the alt-right thinks Carson is a joke. But the mainstream left is terrified of Carson. People like Carson are an existential threat to the modern democratic party, which frankly functions as a religious organization for the modern American leftist. The idea of a major campaign by The Enemy, led by a member of what amounts to their most loyal pet class, is frightening to them. It introduces the possibility that the currently near-100%-locked black vote may end up being up for grabs. That wouldn't be merely inconvenient for the Democratic party - it would be an existential threat.
Now, I don't care for Carson. He's pro-TPP, he's too soft on most things I care about, and he seems exactly like the sort of guy who would turn into a dupe for the ruling class if he did gain power. But take Trump out of the equation and I'd be tempted to back Carson. I wouldn't expect major policy victories from him, but four years of leftists being forced to dump hatred on a black brain surgeon turned politician would be quite a sight.