"Because you knew too much." He simply said. "Unless you vowed an oath to never speak of the things that happened in this manor, you can be forced to confess any information they want from you. Vows such as those need to be made willingly. Dumbledore could also end up in trouble, it is very much against the laws and customs of the Wizarding World to leave children with Muggles once their wizarding parents have died." 