Why did God turn Israel back into the wilderness for forty years?
Because of their unbelief, they did not believe that God meant what He said. At Kadesh they could have entered the promised land without losing a single young son or lifting a sword, but their refusal to trust His word caused God, in His wrath, to turn them back into the wilderness for forty years of wandering.
Les explains that Israel could have entered the promised land directly from Kadesh after leaving Sinai. They could have taken it without losing a single young son and without having to lift a sword. But they did not believe that God meant what He said. Because of their unbelief, God in His wrath turned them back into the wilderness for forty years of wandering. Nearly forty years later, as they prepared to cross the Jordan, they would stop for Passover, and that Passover would be exactly forty years from the Passover in Egypt. Les points out that this shows how meticulous God is with His timing. Deuteronomy is written in that final month or two while they sit opposite the Jordan River, and Moses reflects back and reviews everything since the coming out of Egypt, so much of it is recap.