After the incident in which Moshe hit the rock to draw water instead of speaking to the rock, Hashem gives Moshe a punishment. In 20:12, Hashem tells Moshe, "Therefore you will not bring the nation [into the land of Israel]."
How does the punishment fit the crime? Moshe hit the rock instead of speaking to it. Ok, so he did not maximize the possible Kiddush Hashem. He did not teach them the highest level of emuna he could have. What does that have to do with bringing Bnei Yisrael into the Land of Israel?
Living in Eretz Yisrael is the ultimate level of emuna one can have. Especially after coming from a life of slavery, and the life of the people who grew up in the desert. To live in Eretz Yisrael requires great emuna that Hashem takes care of us and our needs.
If Moshe could not teach Bnei Yisrael that level of emuna, then he is not the right person to prepare them and bring them into Eretz Yisrael. They need someone who can raise them to the appropriate level of emuna.