Well you already pointed at why: because you can be ordered into mental health care. You can’t be ordered into confession, it’s completely voluntary. Furthermore, priests do not have a legal duty of care; they are not registered professionals with professional standards to follow. Their role is defined by the church, not law and regulation.

In a practical sense, such a law isn’t going to work much anyway. It would be almost impossible to prove that a priest had been confessed to, short of someone admitting it directly. So the only way it works is if the child abuser wants to get one over on their priest - giving the child abuser another avenue to hurt someone else.