Why did the Exodus generation have to wander in the wilderness for 40 years?

The Exodus generation wandered for those years because of unbelief; they were not able to enter the Promised Land (Hebrews 3:19). One year after leaving Egypt they reached Kadesh-Barnea, near the border of Canaan, and God sent twelve men to spy out the land. Joshua and Caleb had faith-rest and were ready to go in and take it as God had promised, but the other ten convinced the people they were not able and the enemies were too strong. So God took that generation on a circuit through the wilderness for thirty-nine more years, and every test that came their way was a test of their ability to rest in faith; they failed every time, and every adult except Joshua and Caleb died in the wilderness.