Usually the crops being rotated require completely different nutrients and minerals. Rotating crops allows the soil to recover the nutrients and minerals that have been exhausted by the previous crops. By rotating the crops, the health of the soil is stabilized and balanced.
This wisdom has been around for thousands of years In the Bible (Leviticus 25:3-5) God told His people to use your fields for 6 yrs. and on the 7th let your fields rest. Perhaps so it can regain nutrients that had been sucked out of it for 6 yrs. So the crops that were planted on the 7th year were left to wither and die and be absorbed by the resting soil.