"When the Lord heard what you said, he was angry and solemnly swore: 'No one from this evil generation shall see the good land I swore to give your ancestors, except Caleb son of Jephunneh. He will see it, and I will give him and his descendants the land he set his feet on, because he followed the Lord wholeheartedly.'" NIV translation
Moses reminds the people that their lack of faith led to disobedience which led to their ancestors perishing in the desert. He reminds them that only Caleb had trusted in the Lord and so survived to this point.
"Because of you the Lord became angry with me also and said, 'You shall not enter it, either. But your assistant Joshua son of Nun, will enter it. Encourage him, because he will lead Israel to inherit it. And the little ones that you said would be taken captive, your children who do not yet know good from bad - they will enter the land. I will give it to them and they will take possession of it. But as for you, turn around and set out toward the desert along the route to the Red Sea.'" NIV translation
"Then you replied, 'We have sinned against the Lord. We will go up and fight, as the Lord our God commanded us.' So every one of you put on his weapons, thinking it easy to go up into the hill country." NIV translation
Moses reminds the survivors of the fact that their ancestors had realized that they had sinned against God. Their lack of trust in Him to provide the victory was their sin. So, why did God still punish them and not let them enter the land? They were still thinking that they could take possession of the land through their own efforts. Like the Israelites, we do not have the ability to walk with Him under our own strength.