Why was Moses not allowed to enter the promised land?

[ProDeo]

It's how serious God takes sin.

[Fenris]

I understand what you're saying about the difference in the people, and I can agree that it was time for God to hand the reigns of leadership over to Joshua. But that doesn't explain why Moses struck the rock in anger instead of speaking to it as instructed, nor does it mitigate his accountability. God clearly stated the reason Moses and Aaron would not enter the Promised Land was because they did not do as He instructed regarding the rock:

But the LORD said to Moses and Aaron, "Because you did not trust Me to show My holiness in the sight of the Israelites, you will not bring this assembly into the land that I have given them." (Numbers 20:12)

And there on the mountain that you climb, you will die and be gathered to your people, just as your brother Aaron died on Mount Hor and was gathered to his people. For at the waters of Meribah-kadesh in the Wilderness of Zin, both of you broke faith with Me among the Israelites by failing to treat Me as holy in their presence. Although you shall see from a distance the land that I am giving the Israelites, you shall not enter it. (Deut 32:50-52)

Again, no argument. Just showing you another way to look at the incident.