I have said it till I was blue in the face. The Catholic Church does not have a pedophile problem, they have a homosexual priest problem. 90% of the sex cases against priests are homosexual priests having sex with young teenager boys, who by the way are not pedophiles.
By the way the homosexual clergy problem affects all denominations. The reason? The vast majority of homosexuals are predators. They are always on the prowl for young teenage boys.
Wrt to the question "why was it only boys?" we should note that only boys were allowed on the altar at the time so that was what the priests had access to. HOWEVER, I highly doubt heterosexual predators would be interested in boys. So, we come to the same conclusion: gay infiltration.