That can explain a little. Not the 40% of all corn grown that is used for ethanol.

Which would be better for the nation's security? Having all this ethanol, or having 31x the energy provided by that ethanol via solar production? We couldn't actually use that much solar power right now, but that's part of the opportunity cost: we aren't gearing up to make use of it because we're generating all of this ethanol that we don't need instead! The capacity maintenance argument works both ways: pay to maintain the capacity to grow vastly more corn than we'll ever need, or pay to maintain the capacity to generate tons more energy that we're far more likely to need.

(Also, taking land that has been largely destroyed by industrial corn farming and changing it into land that's growing some more valuable food crop isn't just a matter of changing your mind about what to grow the next year.)