Hi Hully's DS! 
I think the difference between how and why is pretty tricky to work out. One problem is that, although different people in different times and places have different attitudes towards women, I don't think we have much evidence of any society that was entirely un-sexist. It could be that once, women and men treated each other equally and fairly. But we just don't know - it is too far back in the past! So, when we study history, it's often hard to know why there is sexism against women - because however far back we go, we find it is already part of the culture.
Does that make sense? That's why it's hard to answer the 'why', I think.
Some people have suggested one reason might be that when women are pregnant, or breastfeeding small babies, they are very vulnerable. They need someone else around. Now, of course you could say that men are also vulnerable, because without women they couldn't have babies at all ... but of course, if a woman took advantage of a man's vulnerability by choosing not to have his babies, we wouldn't know about her, because she'd have no children and her way of thinking probably wouldn't get passed on to anyone.
If women need men around to protect them when they have babies, I think that may have been when the balance of power tipped towards men. Humans are quite odd, compared with other animals, in that many animal females and animal babies really aren't so vulnerable at this time.