I think that a lot of it is rooted in fear. My personal opinion is that a lot of the oppression of women, not just in Afghanistan but in a lot of places in the world, is rooted in fear of sharing power. When you keep 50% of your population down so far they can never challenge you, then that's 50% fewer potential opponents you're going to have for power in the future. It is a political decision.
It's also rooted in tradition, in the tribal pre-Islamic tradition. Islam is fairly good on women's rights in a lot of ways, but like everything else, it's open to interpretation, and the Taliban chose an interpretation that keeps people oppressed because the further down you oppress someone, the less likely they are to rise up and challenge you.