Jonathan Pie explains it

[Undertoad]

Another Brit has a similar take, Clive Crook: Revenge of the Deplorables

The crucial extra ingredient, I think, was the way the case against Trump was framed. Clinton's goal should have been to detach a slice of his support. The best way for her to do that, issue by issue, would have been to acknowledge the particle of truth in his claims, if any, and say why her approach to the problem was better. Instead, she and her supporters refused to grant the validity of any part of Trump's pitch. Even that wasn't enough. Trump was a racist and a fascist, they said. Support him, and you're no better: Either that, or you're an idiot for failing to see it.

Apparently it takes more than four years of college to understand this: You don't get people to see things your way by calling them idiots and racists, or sorting them into baskets of deplorables and pitiables (deserving of sympathy for their moral and intellectual failings). If you can't manage genuine respect for the people whose votes you want, at least try to fake it.