1. Corn saps nitrogen from the soil. Soybeans add nitrogen to the soil. Since both are cash crops you want to rotate them. Since hemp is a weed and not a legume it would still might have to be rotated in with soy. It's simply not a cut and swap operation like a line of code :)

2. The government isn't incentivizing hemp so now every acre you swap with hemp not only isn't discounted, the demand for it is a fraction of that of corn.

That and it isn't legal to grow in any of the farm belt. Just because you can grow in CA or CO doesn't mean you can in KS or TN.