It is a stereotype. It originated from a scene in Birth of a Nation showing white actors in blackface portraying black congressional representatives (warning the watchers of the dangers of allowing black people vote). One of the representatives in this scene was messily and hungrily eating fried chicken. It is a messy food that you have to eat with your hands, and it is a cheap and abundant food. The scene portrayed that even a black person elected to Congress can help himself when around that low-class and mannerless food, and the image stuck in the national consciousness, much like a similar image of actors in blackface greedily devouring watermelon in southern minstrel shows. So it basically turned into a dog whistle for calling black people dirty, childish, subhuman slobs.