Because women are more important to civilization. Men are meant to provide and care for the family, because they are meant to protect, die, and sacrifice for the family. In Christian marriage, men are icons of Christ and women the icon of the church... which means men, like Christ, should be willing to put themselves in danger for the sake of the women. (also why they open doors, stand, carry things, etc)

This is also anthropologically practical, as women are indeed more important to the continuation of society. Men can go forth and die with a much smaller impact to larger society, but women can only have one baby at a time (whereas men can procreate with multiple if necessary, even though not optimal)