When they had entered Samaria, Elisha said, “O Lord, open their eyes, so they can see.” The Lord opened their eyes and they saw that they were in the middle of Samaria.
Jerusalem committed terrible sin; therefore she became an object of scorn. All who admired her have despised her because they have seen her nakedness. She groans aloud and turns away in shame. ט (Tet)
He left those nations simply because he wanted to teach the subsequent generations of Israelites, who had not experienced the earlier battles, how to conduct holy war.