These people got away with their crimes for so long because many people including parents and assorted acquaintances failed to take the appropriate action. In some cases, it was the police who failed to take action. Think about the victims who reported the abuses at the time who were dismissed and now only getting justice several decades later.
In their defence though, there was less understanding about child abuse and how to deal with it. People certainly knew it was very wrong, but when confronted it with, they were perplexed and unsure what to do next, as there was no culture of ringing the police first and reporting it.