At least as I understand it, it's priesthood --- not leadership per se --- that is a distinctively male charism.
My own hunch (and this is not Catholic Doctrine, it's from Mrs. Don-o's Big Bag o'Hunches) is that women don't function optimally in a hierarchical structure, whether it be the Church, the military, a business hierarchy, or any other. Women's strengths aren't along the lines of the priestly and hierarchical, but rather along the lines of the mystical and prophetic.
All of our illustrious female Doctors of the Church are mystics.
BTW, that same issue overlays everything, including marriage. We are not far from protestants in general not having their marriages recognized by the Catholic church. Taking out the concept of Marriage as a sacrament has some pretty serious consequences for their organization as a whole.