farmers will rotate crops because they leech and release different nutrients from/into the soil. its common to follow up certain crops with beans because they release nitrogen or something into the soil. i'm sure someone else has a better understanding of how it works.
In addition to what Darth_Reagan said, it's for pest control as well. By keeping a plant in the field for more than one season, you provide a food source for pests whose parents went there to feed the previous one. Some diseases only impact certain crops and can stick around in the dead matter only to attack your vulnerable new plants.
If she's the mean girl of agricultural history I image there's also a prehistoric nerds table, planting the 3 sisters in an out of the way field. So they have a place to hang out on their own while getting laughed at for it.
