Hashem spoke to Moses on Mount Sinai saying: Speak to the Children of Israel and say to them: When you come to the land that I give you, the land shall observe a Sabbath rest for Hashem.
Rashi comments on the words; ON MOUNT SINAI. What is the matter of shemmitah [Literally "release". The seventh year of the seven year cycle of laws pertaing to agriculture in the Land of Israel. The laws of the land include "release" of fields i.e., refraining from working them; allowing their produce to be taken by the public; and "release" of debts.] doing next to Mount Sinai? i.e., why does the Torah say "on Mount Sinai" specifically in the context of the laws of shemmitah as opposed to other laws? Were not all the commandments stated at Sinai, But it is written here to teach that just as with shemmitah its general rules, [and its details,] and its finer points were stated at Sinai, so too with all the commandments their general rules and their finer points were stated at Sinai.(end of quote)
Now having said this, there still remains to explain why was shemmitah singled out to teach this lesson in preferance to any of the other mitzvot?
The Rambam states in Hilchos Shabbos (chapter 30,15)"Shabbos and idol worshipers, each one of these is equivalent to all the mitzvot. Shabbos is the eternal sign between G-d and us. Therefore, whoever trangresses other mitzvot is considered an evil person (rishai yisroel). But whoever defiles the (holiness) Shabbos in public has the same status as one who worships idols and the constellations.
The Chinuch (parashas Yitro) further explains that the greatness of Shabbos is expressed by the fact that by resting on the seventh day we are able to free ourselves from all secular and material activities so that we can ponder upon and remember at least once a week that the universe was created in six days and on the seventh day nothing was created. And when we rest on the seventh day we remember that G-d created the universe, because when people rest one day each week others notice and will question them "whats the purpose of this rest?" And the reply will be," Because G-d created the universe in six day and rested on the seventh day".
Likewise, when we observe the mitzvoh of shemittah by resting our fields once every seven years we bear witness to the fact that not only did G-d create the world but He continues to sustain it. Leaving our fields fallow a whole year teaches us that its really not the sweat of our brow that puts bread on the table.
This point is clearly stated further on in the parasha (25\19) "If you will say: What will we eat in the seventh year? - behold! we will not sow and we will not gather our crop! I will ordain My blessing for you in the sixth year and it will yield a crop sufficient for three years. You will sow in the eighth year, but you will eat from the old crop; until the ninth year, until the arrival of its crop, you will eat the old.
To summarize, the mitzvot of Shabbos and Shemittah are both foundations of Emunah, and therefore its appropriate to choose one of these as an example for all the other mitzvot of the Torah.