There are many reasons. I personally believe that women are meant to be mothers, not fighters. If a women felt really strongly that she wanted to go to war, then fine, but I don't think that the US should draft them.
But what if you're a guy and you're shooting at the enemy and the guy next to you goes down, it sounds pretty tramatic, but what if it were a women? How would you react differently?
Women are just different. It sounds sexist, but anyone who thinks that we are all the same lives in there own little world. Science has found men to be more violent and willing to fight, and women more emotional and less willing to fight. Whether this be for biological reasons or becasue of how they were raised, this holds true with almost everyone.