I think there’s something about the parasitic nature of it, taking over an otherwise healthy ear of corn. We tend to think of our edible fungi as growing out of the dirt like a plant, or a fallen tree, or at worst sort of calmy sitting on top of whatever it is using for its own food. THe fact that this has invaded kernels makes them very bad corn kernels and triggers something instinctive. Corn smut is one of those “the first person to try this was in a bad spot” kind of foods.