Radak (6): He sinned and caused the Rabim to sin. He repented, but he could not undo the sin of the Rabim. His evil deeds remained until the land was destroyed due to them. They were not mentioned for him; he was not punished for them, since he repented and humbled in front of Hashem, who heard his Tefilah and returned him to his kingship in Yerushalayim. His deeds were mentioned for those who persisted in them.