Instead of paying farmers not to grow crops why don't we instead subsidize wind/solar arrays on their land?

Because if for some reason a large portion of our farmland becomes unfarmable, we want that land to be aerable for crops. Lets say for example Yellowstone erupts and turns the northwest portion of the United States into a pile of ash, we'll need to increase farm yields in the east and south to feed the survivors and make up for the decreased crop yields due to the mini ice age it would create. Basically it's planning for worst case scenarios. You don't plan an economy for the bare minimum you need, you plan for the disaster scenarios.