I think its because the women stay at home cooking for the men and the men are out in wars. And i think its a bigger intrest for boys to become a ninja then it is for a girl. This has ofcorse not anything to do with "irl", this is just how i think they think in the anime. Just like on a big boat like Titanic wich i think most ppl have seen. "Women and children first" (to the life boats), same here the women and children stay home and the men go out and die in war.

Thats what I think.