>Why is pedophilia so closely tied to Satanism?
Because it is evil. Satanists revere that which is repugnant; full stop.
In more of a mechanical sense, pedophilia is a favorite way for some to blackmail others with and to exchange blackmailable material so that they know that everyone involved is so dirty as to prevent the disclosure of the evil deeds they participate in. Basically, everyone has dirt on everyone involved. The fact that it involves kids is what makes it so bad – nearly all people agree that hurting children is perhaps the most despicable crime against humanity known.