The answer is because the Sabbatical year is one mitzvah which proves that only G-d could be the Author who gave the Torah on Mount Sinai, because it is there that He promises that the year before the Sabbatical will provide enough crops for the next three years (25:20-21). No human being would ever write this law because it would be disproved within six years! The fact that G-d chose to display his control using this commandment also teaches us a lesson about our accomplishments. If G-d chooses to give us more (crops, money or otherwise), He can do so by having us win the lottery where it's obvious that He intervened, or he can make our companies and crops suddenly produce better where we can be tempted to take the credit for the increase. It's up to us to see the bigger picture, and recognize the value of G-d's commitment to those that appreciate Him.