Rashi on Judges
So that Yisroel would be tested through them. This was a different generation, ignorant of the miracles of the Canaanite wars, and not having witnessed the great deeds, they rebelled, and betrayed the Omnipresent.
Metzudat David on Judges
These are the incoming generations that did not know that they should make known all of the miracles that were done for Yehoshua at the time that they fought Canaan. And because of this, they become weak in their faith in Hashem. And for that, these nations were place to test them