Probably because the US is such a religious conservative country. Christianity (especially evangelical and more conservative type), patriarchy and authoritarianism go together, and that’s who the great majority of his voters were.

Considering trump won twice against women but lost when he ran against a man, it seems the majority of voters would rather elect any man than any woman, even if that man is seriously flawed on every imaginable level. Only when both candidates were men were factors other than gender given more weight.