A lack of faith on Moses’ part is the answer given by the Bible (Deut. 32:48-52). Moses was told to command a rock to get water from it and when doing so he hit the rock twice with a rod (Num. 20:7-13).
This might seem a rather extreme consequence given the apparent small nature of the offense, but Jewish tradition has tried to fill in the gaps. Jewish tradition sees Moses as someone who is overly reliant on miracles to accomplish his tasks. When God commanded Moses to lead the Jews out of Egypt he could not convince the Egyptians on his own, he needed miracles from God. Once on the run, he couldn’t out maneuver the Egyptian army and needed God’s miracles to defeat them. And when God asked Moses to command a rock to give water he hit it until water came out of it.
Thus, Jewish tradition sees the repeated striking of the rock as not the reason Moses was denied entry into the Promised Land but a symptom of the reason he was denied entry. The Jewish nation needed to learn to stand on its own two feet (so to speak). Moses was too reliant on miracles and was too easily frustrated when things didn’t immediately go his way. This kind of leadership would not be helpful to the Jewish nation as they attempt to co-exist in the world. Moses’ death immediately before the entry to the Promised Land was a kind of closing of one chapter and the beginning of the next one for the Jewish nation.
Jewish tradition sees this story as a parable of how we are to interact with the world. While miraculous intervention from God is sometimes necessary, it is not to be the sole means through which we convert the world and spread holiness. We cannot beat the world into submission, we must talk with it and God.
