Our Gemara discusses the idea that Shemitta today is only rabbinic and not Biblically required. The source for this comes from a Gemara Arachin (32b) which rules that Yovel (and Shemitta) only apply when all the shevatim occupy Eretz Yisrael.
Although this is derived from a verse, we should reflect on what could be the reason behind this exemption. On the surface, there is value to the Shemitta and Yovel cycle so long as Jews live in Israel. Whether it’s to merely have an occasional year set aside for Torah and spirituality, like a giant Shabbos, or it’s to be reminded that the land belongs to God and not us, as the verse states, “וְהָאָ֗רֶץ לֹ֤א תִמָּכֵר֙ לִצְמִתֻ֔ת כִּי־לִ֖י הָאָ֑רֶץ כִּֽי־גֵרִ֧ים וְתוֹשָׁבִ֛ים אַתֶּ֖ם עִמָּדִֽי׃” Vayikra (25:23). So, why would it not be a mitzvah even with a smaller number of Jews?
The Moreh Nevukhim (III:39) explains one of the benefits of the Shemitta is to allow the land to rest so that it will yield more crops the following year. If so, we can suggest that if the land is not filled to capacity there is less need to give the land a rest, as in any case some areas will remain fallow.
Another possibility is that since the Torah promises that the produce of the sixth year, seventh and eighth year (Vayikra 25:21-22) will be miraculously blessed and abundant, there is a need for the entire Jewish people to be present. That is, since this is such a grand miracle, perhaps it requires the merit of the entire Jewish people. In the absence of that, the miracle cannot be guaranteed and thus the Torah could not obligate observance. Or, one final idea related to this may be as follows. Since this supernatural production requires a great miracle, we need the solidarity of the entire Jewish people to believe in it in order to activate this divine blessing. There is a teaching in Eichah Rabbah (1:33) that when the Jewish people do mitzvos they allow Hashem׳s presence to be strong in this world, but when they slack off then Hashem’s presence becomes weakened. We see from this an important idea in Jewish theological thought; sometimes Hashem needs a critical mass of Jews who believe in Him in order to bring the miraculous blessings down to this world.
Translations Courtesy of Sefaria, (except when, sometimes, I disagree with the translation .)