I actually know the answer to this one, I think: it's because of the Cult of Domesticity, which came in about when the cultural narrative shifted from women being the sinful uncontrollable ones to being the moral guides of society. See, women are moral and pure, in touch with emotions and spirituality rather than with reason and fact, and so their role is to control men's base urges through gentle guidance (as wives), raise moral daughters and guide their sons to the care of other moral women. If a woman's in the "public sphere" she can't do this, and so that has to be left to men, who rule through reason tempered by female nudging.

Thank you, 19th century!