Good question. Anyone?
One possibility is that devout Catholics who are gay have more motivation to become priests than devout Catholics who are straight. If you're straight, and you like sex, the Catholic Church has no problem with that -- just don't become a priest. But if you're gay, you have to keep it all bottled up inside, according to the Church, and becoming a priest might seem like a solution. Becoming a priest also gives cover for their lack of interest in getting a wife and raising a "good Catholic family". As a result, a higher proportion of priests are not only gay, but people who would otherwise be sexually active. And perhaps a higher proportion of devout Catholic post-pubescent boys who would go and confide in a priest are gay .... so, a higher proportion of the molestations are boys .... Finally, maybe for decades it was always considered suspicious for a priest to be spending a lot of time with a young girl, so that was frowned upon, but priests spending time with young boys was thought of as good and proper mentoring and guidance, that was allowed.
These are just guesses, I could be wrong.