Why did God send the Israelites back into the Sinai for 40 years?
Because of their unbelief after the twelve spies were sent into Kadesh, God sent the Israelites back into the Sinai for 40 years of wandering. That judgment followed their refusal to trust Him to enter the Promised Land.
Les teaches that Kadesh was at the upper end of the Sinai Peninsula, where the twelve spies were sent in. As a result of Israel's unbelief following that spy mission, God sent them back into the Sinai for 40 years of what are called the wanderings. He notes it was 39 years plus, and they eventually came out and traveled a circuitous route through Moab, entering the Promised Land from the east, crossing the Jordan opposite Jericho, rather than from the south or west. Numbers chapter 33 lists all their stops and goings.