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.
And many people may have felt that they did take appropriate action AT THE TIME. What may be considered to be appropriate action today may have been viewed very differently 50 years ago.Almost everything was viewed differently......if you misbehaved as a child and were given a clip round the ear by the local bobby, you'd get another one from your dad. Try that today, and both the officer and the parent would end up in court!