The answer to this question is based on understanding the Rambam (in Hilchos Tshuvah, Perek 9). He explains that even though we don’t receive reward for doing mitzvahs in this world, if a person keeps the Torah properly, then HASHEM will remove all of the obstacles that normally prevent a person from keeping the mitzvahs. Sickness, war, poverty, and hunger prevent a person from learning or fulfilling the mitzvahs. If a person is happy and dedicates himself to keeping the Torah, HASHEM will shower him with all of the requirements to better serve Him, including peace, tranquility, well-being, sustenance, and all else that a person needs to follow the Torah.
The Rambam is telling us that since HASHEM created the world in order to have man follow the Torah, when a person uses the world properly, then HASHEM allows him to have his needs met in this world without strain. This will help him better serve HASHEM.
HASHEM was telling Moshe and Aharon that this lesson would have greatly affected the generation of the desert, but it was lost. Had the people seen the rock obeying HASHEM’s command, they would have been moved to a powerful realization: “The rock doesn’t have needs, yet it listens to HASHEM. How much more so should we, who have so many needs? HASHEM has promised that if we follow in his ways, He will remove all obstacles from our path. But if we don’t listen. . .”
That was a lesson that would have affected even this generation because their very survival depended on it. While people may have many lofty motives, one of our strongest drives is self-preservation. Had that generation come to a more clear recognition that their existence was dependent upon keeping the Torah, it would have changed even their appreciation — but it was a lesson lost.
