Based on Be'er Yosef to Leviticus 25:1-3
There’s a Midrash which analyzes 1 a verse in Psalms 2. The verse describes גיבורי כוח, the mighty ones of strength, who follow Hashem’s word. Who does this refer to? The Midrash says that it refers to those who observe the mitzvah of shemittah. Why are these people called mighty ones of strength? If someone one day gave tzedakah 3, it would be commendable, but not overly impressive. If they gave it every day for a week, that would be more impressive. If they gave tzedakah every day for a month, that would be even more impressive. Who would expect anyone to give tzedakah every day for an entire year? That would be tremendously difficult. For sure someone who observes shemittah for an entire year, seeing their land remain undeveloped, and their fruits taken by strangers, and keeps silent, is superhuman 4.
Our Sages are teaching us that the mitzvah of shemittah is incredibly difficult, and requires an enormous amount of strength of character. If so, how could it be that Hashem would expect this from us? Hashem never gives someone a test they cannot succeed in 5, but this mitzvah seems to be too much. Therefore, it must be that Hashem determined that this mitzvah is something that the Jews can handle, just like any other mitzvah. We are taught that Hashem investigated (so to speak) and determined that there was no nation fit to accept the Torah except the Jewish people 6.
Besides being inherently fit to receive the Torah and fulfill its mitzvos, we see that Hashem prepared the Jews for this purpose by bringing them to Mount Sinai. The primordial snake infected Eve with some sort of contamination 7. This implanted in Mankind a sort of predisposition against the Torah and its mitzvos. When Hashem brought the Jews to Mount Sinai, this contamination was removed. Unlike the non-Jews, who didn’t have this experience, this process elevated and sanctified the Jewish people 8 to make them more receptive towards the holiness of the Torah 9. So much so that they were able to precede “We will do” before “We will listen” 10. Most people are not on such a level to be able to have such trust in Hashem to commit themselves to listen to whatever He says, before He even said it. The Sinai experience paved the way for such a possibility 11.
Because the Jewish people were elevated to such a high and lofty level at Mount Sinai, from that point on 12 they didn’t have such a difficulty with the mitzvah of shemittah. To be able to abandon their land for an entire year wasn’t as large a burden. Even though to everyone else, it would appear to be an impossible mitzvah to keep. For this reason, the Torah emphasized that the mitzvah of shemittah was given at Mount Sinai. The very fact that the Jews were brought to Mount Sinai is the reason they are able to keep the mitzvah of shemittah, something no other nation could withstand.
1 Vayikra Rabbah 1:1; Midrash Tanchuma Vayikra § 1; Tanchuma Yashan Vayikra § 1. Eitz Yosef to Midrash Tanchuma understands that Zohar I p. 90a also possibly understands the verse this way
2 Psalms 103:20-21
3 Eitz Yosef to Vayikra Rabbah loc. cit.
4 ואיזהו גיבור הכובש את יצרו (Avos 4:1)
5 Avodah Zara 3a
6 Vayikra Rabbah 13:2
7 Shabbos 146a
8 Rashi ad. loc. s.v. מזלייהו
9 This explains why we mention in the Haggadah in the song Dayeinu that it would have been enough if Hashem just brought us to Mount Sinai, even if he didn’t give us the Torah. This is pointed out by the Abudraham and Kol Bo’s commentary on the Haggadah
10 Exodus 24:7
11 Siach Yitzchak’s commentary to the Haggadah (by Rav Yitzchak Meltzan)
12 Even though we are taught that the contamination of the primordial snake returned when the Jews sinned with the Golden Calf (Nefesh HaChaim 1:6), one explanation is that it returned with a much lower potency. See Ohr Yechezkel Torah VeDa’as for a discussion on this topic. The Be’er Yosef doesn’t discuss this, but he does see from the gemarra in Shabbos loc. cit. that the effects of Mount Sinai are still present. He also brings Nedarim 20a and Yerushalmi Kiddushin 4:1 as a proof. See there. See also UBacharta BaChaim ad. loc. § 22