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.
Most fruits and vegetables can't be stored or transported like grains - unless they are picked before being ripe & at peak nutrition.
Playing games on the supply side is a fool's errand. Incentives become unpredictable, if not counter-productive. Boost demand and incentives will work themselves out. Pay people to live healthy lives and they will drive the farmers to produce healthy foods. Put money on the behaviors you want to see more of - and you will see it. Pay to encourage price stability (farm subsidies) and you'll see price fixing. Pay people who get and stay fit and they will drive the markets to healthier plateaus.
Year-round fresh fruits & vegetables for 300M people can not be grown in the US. You can't subsidize farmers enough for them to make the sun shine longer and breed apples with a 6 month shelf life. You also ought to consider the global effects of shipping fresh food a few thousand miles or more, not to mention the human costs of raising and harvesting all those tasty luxuries.
Grains (and other starches) are stable, travel well (though they can grow closer to everywhere), and can provide an adequate base of energy and fiber within a diet tailored to a healthy individual.
If you want economic incentives done better, tax the fat and transfer it to the fit.
And they don't subsidize things like vegetables.
Now, what do Americans not eat enough of? And it's partly because they're too expensive compared to cheap grains and cheap grain-fed meat?