Apparently some of the bishops who were removed had participated in such behavior years ago. The Vatican, just like a centralized government or the headquarters of a corporation, doesn't always know what's going on in a given diocese. The bishop of a diocese may keep quiet and try to handle such cases himself. Or protect himself from scandal. The thing is the abuse was worldwide, particularly bad in Ireland but pretty bad in many countries, so hard to believe the Vatican didn't know. Probably knew for generations. And was protecting itself.