When God instructed Moses to speak to the rock to draw out its water, Moses did not do precisely what God instructed. Rather, he struck the rock twice. According to the Sages, his doing so in public exacerbated the severity of the matter. Therefore, Moses was severely punished.

“The Lord said to Moses and Aaron: Because you did not have faith in Me, to sanctify Me before the eyes of the children of Israel; therefore, you shall not bring this assembly into the land that I have given them” (Numbers 20:12). Hadn’t Moses once said something more egregious than this? [After God told Moses that He would provide meat for the Israelites,] he said: “Will flocks and cattle be slain for them and it suffice for them? If all the fish of the sea will be gathered for them, will it suffice for them?” (11:22). There too, there is a lack of faith, and it is a greater offense. Why didn’t God decree a punishment upon him then? To what can this matter be compared? It can be compared to a king who had a beloved friend who would rebuke the king with harsh words between the two of them, and the king did not get angry at him. At a later date, he rebuked the king in the presence of the legions and the king decreed that he be executed. Here too…the Holy One, Blessed be He, said to Moses: The first offense that you performed was just between the two of us. Now, it was in the presence of the multitudes; it is impossible to overlook it. (Bemidbar Rabba, Parasha 19)