Those priests got away with it for several reasons. The first is the relationship between the kid's upbringing and the exalted status of the priest. Kids report a lot of gas-lighting by priests who feed them a lot of BS about "special relationships", "God's Love", etc.. Priests are all trained to deliver this sort of crap to their followers, so doing it to kids is easy for them. Remember an average 100 IQ ten year old, is like an adult with an IQ of 67: In other words, a moron. They believe what adults tell them, and they are totally conditioned to believe any sort of nonsense a priest tells them. Their parents do!
The second reason is that their parents don't believe them if they do speak up. Their parents have the same sort of relationship with priests, and all parents (not just Catholics) are aware their kids are perfectly willing to lie to get out of trouble.
The third reason is a church hierarchy that values a priest far more than a few dozen molested kids, and believes in redemption. Their superiors have all experienced sin and forgiveness in their lives, so they forgive and accept a promise to "sin no more". Then they move the redeemed priest off to a new parish so he can molest a fresh crop of kids. And of course, in some cases the superior did the same thing himself when he was a new priest.
The fourth reason is the rule that all organizations try to hide stuff that reflects badly on the organization. This is why we need mandatory reporting, and conspiracy prosecution for anybody who fails to report any kind of sexual assault to the police.
The fifth reason is that a lot of the police and prosecutors may refuse to arrest or prosecute priests for religious and cultural reasons.
When you have all those things going against you, it's easy to just keep silent and try to forget about it.