It's called "the cult of personality." Somebody manages to grab onto the Zeitgeist and runs with it. He acknowledges and validates the fears of the most fearful half of the population, and gives them an object upon which to focus their hate.
Hitler and the Jews. George Wallace and blacks. Cromwell and Catholics.
Pathologically needy, terrified people will deny facts before their eyes and reasoning a five-year-old could master as long as the Dear Leader tells them they're wonderful, and the reason they're not kings of the world is because of the treachery of THAT identifiable group ("Mexicans takin' our jobs!") and the evil, perfidious traitors who enable them (Jews, liberals, educated people, whatever).
It's not even about intelligence. It's about emotional needs and chronic insecurity.
Religion works the same way.