Because it is consistent with OT and NT that Jesus was uniquely God-man, because without being fully God and man. God has no part in the sacrifice, but when God is the one being sacrificed through Jesus it makes much more sense. Think the binding of Issac. Also that God would make good His promise of redemption with Himself by His own hand. It also solves the problem of evil, in that "why does God allow bad things to happen?" - who answers for God allowance of imperfection only God can make that kind of sacrifice. He died to reconcile a creation broken by allowed autonomy to Himself.