Because a black man in the White House would shock the nation and maybe even ignite a race war. Shades of Charlie Manson and Helter Skelter. But then, Manson was a white supremacist too. And equally not in touch with reality.
This demented reasoning isn’t just on the right. You see it sometimes on the Left too. If the repression gets real bad and the economy collapses, then the masses will rise up in righteous wrath and join our cause.
(Why they will join the cause is never really explained. You could make an equal case they will rise up, blame the Left or the Right, then shoot them.)