Why are a vast majority of pedophiles men?
Good question. I think that as many have suggested the answer is likely to be complex. But I'll tell you my own impression.
Men are more driven by sex. There are sound evolutionary reasons for this. A man doesn't have to invest as much in his offspring as does a woman, and so is less selective of his sexual partners (though perhaps as selective of his long-term mate).
If a man can impregnate someone else's mate without that someone else knowing about it, he's managed to do what the kukoo bird does - his child is likely to be given resources, but at no cost to him. We see this sort of thing happening in birds all the time, and I find it likely that there are similar forces at play in human biology and evolution.
Because of all this there tends to be more competition amoung men for mates than amoung women. The guy who has more sex with more women will tend to leave more offspring.
Of course there are other factors at play. Being a good father, careing for your children, educating them to the necessities of life, etc. all pay off. The problem amoung men is that they are less certain than women that the child who they are devoting resources to is their own. (also, because the woman has already devoted resources to her offspring at birth, the man can get away with letting her continue to devote more than he does, while he goes off and woos the girl down the street).
And this, I suggest, leads to men
Even if you don't like my above argument, it seems hard to attack the idea that men are more driven by sex (that doesn't mean they like it more, gain more pleasure from it, or anything, just that searching for new and more sexual partners is the driving force of more of their behaviors) than women.
When for whatever reason, sexual attraction and arrousal moves toward young children, it seems that this is again more likely to drive male behavior than female behavior.
Women may be as likely to be sexually attracted to young children as men, but less likely to act upon that attraction.
Similarly, men tend to be more violent than women. As such, violent pedophilia should also be more common amoung men than women.
One more factor to add to my list of speculations:
Men are fertile for longer than women. While both men and women decline in fertility as they age, only women go through menopause. Older women are much less likely to become pregnant, carry the child to term, and survive childbirth. If she dies, she can't support her (and the man's) offspring, and the child will likely die (at least in the environment in which we evolved). If a man has sex with such a woman, he has much less to gain (reproductively speaking) than he does with a younger, but fertile, women. Thus, it pays older men to be attracted to younger women more than it pays older women to pay attention to younger men.
Particularly because women go through the dangers of childbirth, when they get older, sex is dangerous. There comes a point where the chance of having one more child (who may not survive anyway) is not worth the risk of dying and no longer being able to help support her surviving offspring (including grandchildren).
This suggests that men should be attracted to young but fertile women (which I think is true), but it's possible that this sometimes gets perverted to drive the attaction simply to younger and younger girls.
As I said, mostly idle speculation. Take from the above what you will.