One reason could be the strict control by the official copyright holders. They might be worried about fanfic diluting the official brand or misinterpreting the original story. Another factor could be some fans' purist view. They only want to stick to the original works by J.K. Rowling and consider fanfic as something not in line with the 'true' Harry Potter universe.
Well, it could be related to the fear of changing the established lore. Harry Potter has a very detailed and beloved world. Fanfic sometimes takes creative liberties that some fans find unacceptable. For example, changing the relationships between characters in a way that goes against how they were presented in the books. Moreover, there could be a perception that fanfic is a form of 'unauthorized' storytelling that doesn't deserve the same attention as the official works.
For fans, it means a more restricted creative outlet. Many fans who enjoy writing fanfiction might feel disappointed as they can't freely explore their ideas within the Harry Potter universe.
The shunning of Harry Potter fanfic has a significant impact on the fan community. It can discourage new fans from exploring different aspects of the Harry Potter world. Newcomers might see the negative attitude towards fanfic and think that they should only focus on the official materials. This can lead to a less diverse and inclusive fan community. Additionally, it can affect the way fan - made content is perceived in general. If fanfic is shunned, other forms of fan - created works, like fan art or fan videos, might also be undervalued or not given the proper attention they deserve.
One possible reason could be extreme despair. Maybe in that fanfic's alternate universe, Harry has lost all his friends and loved ones in a very tragic way, like a second wizarding war that was far more brutal and unforgiving. He might see no way out of the emotional pain and thus choose to end his own life.
It could also be that the fanfic might interfere with the way he wants his story to be remembered. His adventures at Hogwarts were real and important to him, and when fanfic changes the narrative, it's like his story is being disrespected. For example, if a fanfic portrays him as cowardly when he was actually brave, that would surely make him dislike it.
One reason could be a personal betrayal. Maybe a close friend from Gryffindor betrays Harry, and this makes him start to dislike the whole house. It's like when one bad apple spoils the bunch in his eyes.
Another reason might be jealousy. If a character in the fanfic is trying to do something related to Harry Potter, like write a book about him or start a movement in his name, others who are jealous of their potential success could send rejection letters to thwart their efforts.
One possible reason could be extreme depression. Maybe in the fanfic, Harry Potter has faced so many losses and hardships that he feels there's no way out. For example, if he lost all his friends and felt alone in the fight against dark forces, it could push him to such a dark place.
One possible reason could be a miscommunication. Maybe Hermione accidentally reveals a secret of Harry's that he didn't want known, and this causes him to feel betrayed and start to hate her. Another reason could be jealousy. If Hermione gets more attention or praise for her magical abilities than Harry in the fanfic, he might develop feelings of envy and hatred. Or perhaps there's a love triangle situation where Hermione is involved with someone Harry doesn't like, and that causes his hatred.
One reason could be exploring dark themes in the Harry Potter universe in a more extreme way. Some fans might be interested in delving into the psychological struggles that could potentially exist in that world, even if they are very dark. Another reason could be a form of catharsis for the authors, who may be dealing with their own internal issues and using the fanfic as a way to express their feelings in a fictional context.
One possible reason could be a misinterpretation of his character. In the original series, Harry Potter doesn't hate Muggles at all. But in this fanfic, perhaps the author wants to explore a 'what if' scenario where something drastic has happened to make him have such feelings. Maybe he's been betrayed by a Muggle - in the fanfic world, this could be a made - up situation that sours his view on all Muggles.