In his discourse to Bnei Israel before they entered into the Land of Israel Moshe relates the incident of the spies and how they spoke negatively about the land. Then he relates that they admitted their fault and they did Vidui, they asked for forgiveness. Moshe says: VATOMRU ELAI CHATANU, “…and you said to me ‘we have sinned’… ” (Deut. 1,41) They were then prepared to go forward and to enter the land but Hashem told Moshe to prevent this action. He did not accept their confession.
We know that Teshuvah has to be done with a full heart. If it is said verbally but not truly felt it is not only meaningless and not acceptable but is really an additional transgression. That is why on Yom Kippur there is a special AL CHET in which we say: …SHECHATANU LEFANECHA BEVIDUI HAPEH, “…for having transgressed by verbal confession or insincere confession.”
This is what Moshe said to the spies, you confessed ELAI, to me but not to Hashem. Your Vidui was only in your speech but not in your heart. It was not a true one and hence it was not acceptable.