A lot of people are racist, hateful, bigoted, and these figureheads give them permission to be their worst selves. That's really the long and the short of it.
That's seriously the only valid explanation.
People used to be shy about their bigotry out of fear of being seen as terrible people.
Folks like Trump or Le Pen basically gave them "permission" to be ABSOLUTELY LOATHSOME folks and to feel part of a movement of like minded people guilt free.
As close a thing to "rational" as they come is that they believe that a racial hierarchy is a good thing, what's gone wrong in their culture is that too many of the "Wrong" people are getting things they don't deserve, and the way to fix it is to be awful to them
We do not want to encourage this
Maybe I'm naive, plenty of white folk who aren't racist "in their hearts" don't know how racist the policies they support are.
The Southern Strategy didn't fool Northerners into thinking Southerners aren't racist - it fooled the Southerners's kids.
But I'm teachable about this if I'm wildly wrong.
Most Americans (by a huge preponderance) are dysfunctional and yearn for bullying leaders to tell them what to believe and who to hate and that is Trump