I also though about this question a lot. I think it�s because when a boy goes off the derech or slips in his Yiddishkeit it takes a lot less nerve. When a girl stops being Tznius she had to actively go buy a short/tight skirt whereas a boy just accepted a cigarette from a friend or acted rowdy. Maybe because it�s in a boys nature to do non Toradik things? I don�t fully understand.
