Parashat BeHar begins with the laws of Shemitah by first stating that Hashem spoke to Moshe at Har Sinai. The Pesukim then go on to list the various laws with regards to Shemitah and Yovel. Rashi asks the obvious question: What is the importance of stressing that these laws were given at Har Sinai? Rashi answers by saying that this teaches us that just as the laws of Shemitah were given at Sinai, all other Mitzvot were also taught at Sinai. However, Rashi’s answer doesn’t tell us why Shemitah was chosen out of all Mitzvot in the Torah to teach us this principle.
The Lubavitcher Rebbe said that out of all the Mitzvot given to the Jewish people, the prohibition of working the ground during the Shemitah period is one of the last precepts to be implemented. Since this Mitzvah is an agricultural Mitzvah, it came into effect only after the Jewish people entered Eretz Yisrael, forty years after the giving of the Torah. Even then, many of the laws pertaining to the land did not fully come into effect until the land was completely conquered and occupied by Bnei Yisrael, fourteen years after entering the land. Even at that point, Shemitah would take place only seven years later, since it occurs only after six years of work. Therefore, when the Torah was given at Har Sinai, one can imagine the significance of the Mitavah of Shemitah in the minds of Bnei Yisrael. This Mitzvah was, at the time, one of the least relevant Mitzvot to Bnei Yisrael. The Torah is trying to teach us a valuable lesson. It is because of this very point that the Torah teaches us that Shemitah was said at Har Sinai; since if Shemitah, the least relevant Mitzvah, was taught on Sinai, then obviously the other more relevant Mitzvot were also taught.
There remains, however, one small problem with the answer given above: Why not use Hakheil, the gathering of the entire nation to hear the Melech read portions of Sefer Devarim, which was to be done the year after Shemitah, as the precept to convey this principle? If the Torah selects the least relevant Mitzvah in terms of how much time it would be until the Mitzvah takes effect, why not use Hakheil?
There is a major difference between Hakheil and Shemitah. Although it is true that Hakheil is further away than Shemitah with regards to the chronological relevance of the Mitzvah, conceptually, Hakheil is much closer. One of the reasons for Hakheil, as stated by Rambam (Laws of Festival offering 3:6), is to remind us of when we gathered at Har Sinai. Therefore, despite Hakheil being distant with regards to time, it is actually very close in terms of its connection to Har Sinai. This is contrary to Shemitah, which is both distant in time and concept. The fact that the Mitzvah of Shemitah, which was far away not only in time, but also in concept, was taught at Har Sinai, shows that surely all other Mitzvot would have been presented there as well.