Flip it around. Why is subsidizing corn a good thing?
The USA doesn't want to be dependent on other countries for basic goods like food; imagine if there was an "OPEC" of food in 1973 and they decided to cut off the food supply to protest US support of Israel.
The exception are soft fruits like strawberries, avocados, or apricots, where harvesting them mechanically will bruise them beyond what a consumer's willing to eat. These are luxuries, not staples, though. Things like wheat, corn, and other grains get harvested by giant machines and require little labor input.
But the US doesn't subsidize labor-intensive crops.