The Israelites chose what would seem to be repentance. They proclaimed, "Here we are, we shall go up, for we have sinned." They are ready to go into the Promised Land, they will follow God's command. But Moses warned them to cease, because "You should not be routed by your enemies, because God is not with you."
God had told Moses not to allow the people to enter Israel. He had sworn an oath that they must wander in the wilderness for 40 years, until the adult generation that sinned with the spies had passed from the earth. Once God has taken an oath, there is no option for nullifiication. NOT precipitating an entry to the land becomes a Divine commandment. Thus, even though it would seem that the Israelites wanted to repent, by proceeding to attempt to conquer Israel they are committing a new sin.
This moves the goal posts, and brings the question on to God. Why did He swear that they must perish in the wilderness? Why not leave an opening for repentance, and why not embrace the children of Israel's desire to correct yesterday's wrong?
I would like to suggest two answers. The first relates to the manner in which the Israelites, newly freed slaves, connect to the rest of the world. Sovereignty requires maturity, and the ability to carefully weigh all options before making decisions. To run your own country, and run it stably, is not easy. It requires a steadiness and a commitment to foundational values.
The history of the Israelites in the wilderness was the opposite of this. Every time they camped, there was a crisis. When there was no water, people already cried out that they wished to return to Egypt. In the section prior to this story in the Torah, the Israelites resented the manna that was their food, and desired the delicacies of Egypt. Going back to the golden calf, the moment Moses seemed to delay his return, the people panicked and demanded that that calf be created. What is the common denominator of all of these stories?
A lack of stability, and emotional volatility. If this is the way that the people run their affairs, sovereignty will be a disaster for them. They will be completely vulnerable to fear and weakness, and to the seduction of the pagan ways of life surrounding them. In other words, they might quickly sin, thus forfeit God's protection, and hasten their exile.
Modern history has taught us the dangers of granting sovereignty to a volatile people that is not ready for it. When the United States invaded Iraq, many believe that by setting up a democracy, a Middle Eastern civil society would emerge. This naive approach ignored the deep ethnic hatreds and primitive ways of dealing with them that were prevalent in the lands of Iraq. By putting on the outer garment of free elections, the West fooled themselves into believing that the people wearing those garments would be transformed. It did not work that way, and Iraq today is an anarchic mess.
Perhaps for this reason God wanted the Israelites to remain in the desert, even if their next-day-repentance was sincere. God loves his people, and wanted them to be completely ready to enter the holy land. One could say that the spies episode was a test of that readiness. If the people had a steadiness of faith, they would not have been moved by the fear mongering of the 10 spies. They might have said, "It's a challenge, but God will be with us and we can meet that challenge." Had they done so, they would have proven that they were no longer subject to the winds of emotion.
A second, complementary reason for God's desire not to allow the Israelites to succeed on the day after is the lesson of the day after itself. In other words, God wanted to teach them how to deal with failure. After all, the entire exodus has been leading up to the entrance to the land of Israel, and the spies were the first part of making that dream a reality. It blew up in their faces. How do you deal with that?
God sees into the hearts of men, and knew that the repentance the people expressed on the day after was really Freudian denial. What had happened yesterday caused them such shame, that they wished to make believe it had never happened. They stood before Moses and said, "Here we are." Whatever happened yesterday was somebody else. We are here.
The danger of denial is that it prevents identifying and healing the flaw that is being denied. Something was fatally wrong with that generation, and had the Israelites been able to go into Israel the next day, that fatal flaw would have remained untreated. As we have said, that flaw was an instability, and emotional volatility.
King David said, "Yay though I walk through the valley of the shadow of deepest darkness, I fear no evil, for You are with me." There are two concepts that are crucial to the religious person's identity and success: Belief and Faith. Belief means knowing that God exists. The Israelites were certain of God's existence, their belief was complete. After all they had seen in Egypt and at Mount Sinai, it would've been nearly impossible to lack a strong belief.
Faith, though, is another story. Another word for Faith is Trust. Trust that God will protect, trust that God's instructions are good, thus that God has our interests at heart. King David was expressing trust, that no matter how terrifying the world around him became, he knew that God was with him. As a result, he concludes the psalm by saying, "Only goodness and kindness shall pursue me all the days of my life, and I shall sit in the House of the Lord for the length of days."
King David was the greatest sovereign of the entire monarchic era. That was thanks to his Faith, his trust. It was this kind of Trust that was missing from the desert generation. Without it, they would not have lasted as a sovereign nation in the land of Israel. God saw that they needed the time in the desert to gain this trust, to follow God wherever He led them.
"I have remembered the kindness of your youth, as you walked behind Me in the desert." God wanted a new generation to enter the land, a generation that had grown up with this level of Trust. Perhaps the word "youth" might be taken literally, to refer to the children of the Exodus generation, who would be the ones to enter and build sovereignty in the land of Israel.
A third, additional answer, thus emerges. God wished to show this new generation that His word, when accompanied by an oath, will be fulfilled no matter what. If his oath to keep them in the wilderness would be fulfilled, then his promise to bring them into the land of their fathers and bless them would certainly be fulfilled. With that faith, they would be ready to assume the responsibilities of sovereignty. They would be stable, they would be firm, they would be unafraid and unaffected by the threats and temptations of the peoples around them.