Why were the Israelites unwilling to enter the Promised Land?
They were unwilling because they had grown weak in faith and forgot the power their God had already manifested. Rather than trusting God's command to go up and possess the land, they requested spies; the spies' report then made Israel afraid, and they concluded they could not do it.
Les explains that in Deuteronomy 1 Moses is reviewing the journey from Horeb, or Mount Sinai, through the great and terrible wilderness to Kadesh Barnea. God told Israel the land was set before them and commanded them to go up and possess it, and He did not tell them to send in twelve spies. The idea to send spies came from the Israelites themselves, because they backed off, were weak in faith, and forgot all the power their God had manifested. They said they could not go in until they knew what they were up against. God condescended, but Les notes that Israel got weak and afraid because of the spies' report and said, "Oh, we can't do it."