Because God is God
“In the last resort forgiveness is always due to God’s being what he is, and not to anything that man may do. Because God is God, he must react in the strongest manner to man’s sin, and thus we reach the concept of the divine wrath. But because God is God, wrath cannot be the last word. ‘The Lord is good; his mercy endureth forever’ (Ps. 100:5).”
Leon Morris, The Apostolic Preaching of the Cross (Grand Rapids, 1965), page 154.