‘ So Yahweh left those nations, without driving them out quickly, neither did he deliver them into the hand of Joshua.'
Here the writer makes plain the truth. Yahweh had known all along that His people would be unfaithful, for even in the days of Joshua when the people were relatively faithful to Him, He had not acted fully to drive out the nations with all speed. This was so that they would be a test to His people of their faithfulness, a test that they had miserably failed. He had been sovereign over affairs right from the beginning.
And yet, on the other hand, part of the reason for their not being driven out, as He has made clear, was because of the refusal of His people to drive them out. They had done so at first, but then they had slackened off. And as time past they had even made deals with them, becoming their taskmasters, receiving tribute from them, socialising with them, learning their sophisticated ways, when all the time they should have been concentrating on driving them out. Thus they had contributed to their own testing. This recognition of the fact that man's failure was within Yahweh's sovereignty is a feature of the historical prophets from Joshua to Kings, for everything was within His sovereignty.
It is also a picture of the Christian life in which Christians again and again compromise with sin and worldliness instead of driving them out and then wonder why they are till entrapped by them.