Moses was supposed to speak to the rock in front of the people; instead Moses spoke to the people and hit the rock. Big deal! Why was God so upset about that one little thing that it meant Moses was forbidden from entering the Land (v 12)?
We don’t know what he was supposed to say to the rock, but presumably it would have been about God commanding it…. The people would have heard God-talk and then seen what was an undisputed miracle.
Instead, in what he says, he implies that it’s Aaron and himself that are doing this “Must we bring you water from this rock?” And then when he strikes it, and water comes, the miracle is lessened—the people might think he is a very smart guy who knew that this rock was a natural barrier holding back a natural supply of water; crack it and out the water will come. He has 1) obscured the connection with God and 2) made it less clearly a miracle. Thus God says “Because you did not … honour me as holy in the sight of Israelites….”
God is not upset about rough treatment of rocks, God is concerned that the people learn to trust him.