Another example of the adage, "Power corrupts, and absolute power corrupts absolutely."

Any institution that gives absolute power to adults over children, is likely to find some of those adults have abused their power to abuse those children.

Priests in particular have had pretty much absolute power over children, because parents trusted them absolutely, and so did the children. The power structure of the church is based on the assumption that priests are divinely called and therefore have divine authority, which makes it particularly difficult for the hierarchy to admit they could abuse that power.