Because people are miserable and desperate and they want to blame someone or something, and bigotry is simple and superficially satisfying.
And because some number of those who actually are to blame for their misery and desperation have self-servingly encouraged them.
I sincerely have no idea.
The narrative that a leftist couldn’t win is repeated so predictably and so often and by so many people that the whole idea has become sort of detached from reality, and there’s no telling what would happen if it was actually a possibility.
And particularly since the one thing I’d pretty much guarantee is that the concerted efforts on the part of the ruling class to prevent a leftist from running would be as nothing compared to what they’d do and say in order to prevent one from winning.