I think there are a few things to consider:

Looking at the Church globally, there are comparable rates of abuse in many cases in other institutions, and just in general. Huge numbers of people have been sexually abused (in the States, statistics suggest one in every five girls and one in twenty boys, and 28% of youth by the age of 17: see link).
In certain regions, the rate of abuse in the Church seems higher than average. There were groups of priests who would systematically abuse children, and bishops were sometimes part of these groups. So there is/was definitely a real and extraordinary problem in some areas of the world.
There was often an attitude in the Catholic Church that such sins were the result of a one-time temptation and that once the perpetrator of the crime had reconciled with God and the Church, promising to reform himself, there was no reason to disbelieve him, and he could continue his ministry.
It used to be a lot easier than it is now to get through seminary and become a priest. Rigorous screening and psychological examinations were not required of seminarians.
Gay Catholic men would often become priests rather than reject their faith or marry a woman they were not attracted to. Seminary formation did not include lessons on the dangers of sexual repression, so many gay men made it into the priesthood with little ability to master their sexual urges. Priests were well respected, and thus many straight men also became priests for the wrong reasons, and were not always able to control their sexual instinct after a while. This accounts in large part for the 50% of abuse cases in which the victims had reached or finished puberty. The abusers were men who broke down under pressure after a while and gave into temptation, since they unhealthily repressed their sexual desires.
In the other 50% of cases in which the victims were pre-pubescent, the explanation that most readily comes to mind is that pedophiles would naturally have been attracted to the priesthood. After all, who would have been among the most trusted members of any community before the sex scandals? The parish priest. Becoming a priest gave ready and frequent access to children, and put one in a position of authority over them, making it unlikely for one to get caught.

Thus, the abuse scandals in the Church seem to me to be the result of (a) poor seminary formation, (b) poor screening of potential priests, © misguided mercy and forgiveness, and (d) already evil men entering the priesthood with the intent of abusing the accompanying privileges.