To be fair, the subsidies exist (if only theoretically) to ensure that America can feed itself in a large scale war. In WWII, the Allies had a lot of success with cutting off Germany's external trade, which significantly reduces their ability to make war. America wants to make sure that if that happens to them, there won't be a famine.

Vegetables aren't a priority in that situation, because they can be easily grown in backyard gardens. On the other hand, row crops tend to be carbohydrate-heavy (perfect for a famine), cheap to produce, and they benefit significantly from economies of scale.