All plants require different levels nutrients to grow. If the same plant is grown repeatedly in the same soil then the soil will run out of the nutrients that plant needs and growing that plant becomes difficult. By rotating through plants with different nutrient requirements, the soil can maintain a sustainable balance of nutrients.

We now use the scientific method to argue ideas, but in the past ideas could just be laughed at if people thought they sounded dumb. People laughed at ideas like the sun being the center of the solar system and doctors needing to wash their hands before surgery. Refusal to accept these ideas held humanity back from technological advancement.

You can't really grow the same crop in the same field season after season (without fertiliser), because they'll sap the specific nutrients they need from the soil. If you do that over and over eventually the soil wont have any food for that crop. Growing something different each season that takes different nutrients from the soil lets it recover the other ones. I don't know how it recovers on its own, circle of life stuff probably. Modern farming can cheat by artificially replenishing the nutrients with fertiliser.

Unobservant idiots will always try to hold progress back.

As others have said, a monoculture will drain those specific nutrients.

Beans are kinda special being a nitrogen fixer. Lot of plants will drain nitrogen so we fertilize as supplement.

Plant beans, squash, and corn together and you won't need to rotate your fields and the plants will form a sort of symbiosis working together.