There are several reasons. First, the Harry Potter series is a beloved and iconic piece of literature that has influenced generations. Promoting 'cannibal Harry Potter fanfic' would taint its positive image. Second, it can have a negative impact on the mental health of those who come across it, especially the young and impressionable fans. Third, it violates the ethical and moral standards that are expected in the fan community. Fanfic should be a creative expansion of the original work in a positive and respectful way, not something that involves such dark and immoral themes as cannibalism.
Well, 'cannibal Harry Potter fanfic' is just not suitable for promotion. The Harry Potter franchise has a large and diverse fan base, including many young readers. Promoting such disturbing fanfic would expose them to inappropriate and potentially traumatizing content. Moreover, it disrespects the original work which is meant to be a story of magic, adventure, and the triumph of good over evil. It takes the characters and the world created by J.K. Rowling and twists them into something unrecognizable and unacceptable.