Because the Church teaches so and the Church knows best.
I have posted several Papal teachings in this regard in the other thread. The realm of woman (most especially if she is a wife and a mother), is home.
Contrary to modern and feminist belief, it is physically impossible to be in two places at the same time. How is a woman supposed to make home and tend the children, if she is outside?
I think the confusion among trads on this point is the question on wether it's ok to work outside of the home after being married but before children. Most agree that working outside the home with children is a no-no, but see no problem with doing it before the first child comes along.
My husband settled that question before it even became one when we were engaged. He made it clear that I was not to work outside the home once we were married, even if we were so unfortunate as to not be able to have kids for some reason. I was so relieved! It gave me every opportunity to be single minded on making the home.