One possible reason could be that the owl got lost on its way. Maybe it was attacked by some magical creature or got distracted by a powerful spell. Another reason might be that someone deliberately intercepted the letter. In the wizarding world, there could be dark forces at work who don't want Harry to go to Hogwarts. It could also be a bureaucratic mix - up at Hogwarts itself. Maybe the admissions office made an error in their records and didn't send the letter.
The story might progress with Harry becoming bitter about never getting the letter. He could start to resent the wizarding world. However, as he grows older, he might start to notice strange things happening around him that he can't explain. Eventually, he could start to investigate on his own and find out about Hogwarts and the wizarding world in a more roundabout way. This could lead to him having a more complex relationship with the magical community compared to the original story.
In the fanfiction, it could be that Harry's aunt and uncle managed to intercept the letter before it got to Harry. They were so determined to keep Harry away from the wizarding world that they might have found a way to stop the owl or trick the postal system. They could have used some kind of Muggle - magic repelling charm they got hold of somehow. This would be a very interesting twist in the story as it shows how far they would go to keep Harry ordinary.
Maybe he leaves because of a conflict with the Hogwarts staff. If the new headmaster or some teachers start implementing rules that Harry believes are wrong or unfair, like restrictions on defensive magic classes, he might decide to leave in protest.
One possible reason could be that he wants to break away from the pre - determined path. In fanfic, the author might want to explore an alternative version of Harry who is tired of the constant danger and expectations at Hogwarts. He may feel that staying there will only lead to more battles and losses, so he rejects it to seek a normal life.
Maybe it's a magical protection. In the wizarding world, there could be some ancient magic associated with hair length. Long hair might act as a sort of shield or a connection to some deeper magic that Harry unknowingly benefits from.
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 possible reason could be that he's tired of the constant danger at Hogwarts. There are always threats like Voldemort's followers or dark creatures lurking around. Another reason might be that he feels pressured by the expectations placed on him as 'the Chosen One'. Everyone expects him to save the day all the time, and it's a huge burden.
One possible reason could be misinformation spread by the Ministry of Magic. They might have spread false rumors about Harry being a threat, which influenced the Hogwarts community. For example, if the Daily Prophet printed false stories about Harry's supposed dark magic activities, it could turn some students and staff against him.
One possible plotline could be that Harry discovers a hidden magical world outside of Hogwarts. Maybe he stumbles upon a group of rogue wizards who teach him their own brand of magic, away from the traditional Hogwarts curriculum.
Maybe he feels that the rules at Hogwarts are too restrictive and are hindering his ability to truly learn and grow as a wizard. He might think that the traditional teaching methods are not suitable for him and his unique style of magic, so he rebels against the system.