Judges 3:1
Now these are the nations which the LORD left, to prove Israel by them, even as many of Israel as had not known all the wars of Canaan; Now these are the nations which the LORD left, to prove Israel by them, even as many of Israel as had not known all the wars of Canaan;
The Lord deliberately left the nations in Canaan in place rather than removing them, in order to prove Israel by means of the challenges that remained among them. The continued presence of those peoples was not oversight or inability but intentional testing, allowing difficulty and opposition to reveal character and faithfulness. That testing principle is not confined to ancient Israel but operates in every generation. God brings His people back again to the very points at which they have failed, and when they fail a test He brings them around to that same test again until it is met.