Darwinian theories about women being wired differently than men are perpetuated because science backs them up. We don't know exactly how, or how much of the difference is genetic vs. cultural, but it's just a reality that men and women have different motivations, aptitudes, etc.
Nobody would argue that evolution has made us different physically. Men are much stronger and better coordinated. Women are more able to bear pain and often have stronger senses of smell and taste. Why would it not stand to reason that evolution might make each sex better at some mental tasks than the other, just as it has physical? It's not sexist to point out that male weight-lifters are able to bench more because men have been designed by evolution to be stronger. Why is it sexist to say we show up more frequently in science departments because we have also been designed by evolution to be better at math?
Sexism is not believing that one sex is different than another, it's believing that those differences make it inferior.