One reason could be disillusionment. Maybe he discovers that the wizarding world is not as noble as he once thought, with a lot of hidden political intrigue and corruption. For example, the Ministry of Magic might be misusing its power, and Harry, being a symbol of justice, can't stand it anymore.
Another possibility is to protect his loved ones. He might believe that the only way to keep his friends and family safe from the constant threats in the wizarding world is to remove himself from it completely. He could think that as long as he is in the wizarding world, they will always be in danger.
One reason could be that he values the authenticity of the wizarding world he lived in, and fanfiction often strays from that.
In some cases, he could be leaving in search of a greater purpose. He might feel that there are things he can learn or achieve outside the wizarding world that will ultimately help the wizarding community more. He may also leave because of some prophetic vision or a feeling that he doesn't belong there anymore, that his path lies elsewhere, away from the familiar magical realm he grew up in.
One possible reason could be the trauma he has faced. Harry has endured a great deal of pain, from the loss of his parents to the constant battles and threats in the wizarding world. He might want to escape all that and find peace away from it.
One possible reason could be extreme anger. If something really bad happened to his loved ones, like if Voldemort came back in a different way and killed all his friends and family this time, Harry might be driven to the edge and decide to destroy the wizarding world that couldn't protect them.
Maybe Dumbledore discovers some dark secret about Harry's past that he can't handle. It could also be that Dumbledore is trying to protect Harry in a really strange way, by pushing him away from the dangerous wizarding world around Hogwarts.
In certain fanfiction scenarios, Harry could leave the wizarding world to explore other magical realms or non - magical places. He may feel that his growth and understanding of magic and life are restricted within the wizarding world he knows. Maybe he meets a group of people from outside the wizarding world who offer him a new perspective and a chance at a different kind of life.
Perhaps he feels betrayed by the wizarding world. Maybe some of his friends turned on him or the institutions he trusted failed him. For example, if the Ministry of Magic wrongly accused him again, it could push him to abandon it all. He may also be haunted by the many deaths he witnessed and feels that leaving is the only way to escape the painful memories.
Abandoning a novel can happen due to changes in the writer's life situation. For example, a major life event like a move, a new job, or a family emergency can disrupt the writing process and make it difficult to continue. Additionally, if the writer discovers that the market for the type of novel they're writing is oversaturated, they might feel discouraged and abandon it, thinking that it won't get the attention it deserves.
Perhaps he leaves in search of a more peaceful life. The wizarding world, despite its magic, has been filled with turmoil for him. He could be seeking a place where he can simply be Harry, not 'The Boy Who Lived' or the hero who defeated Voldemort. He may also be on a quest to discover more about his family's past in the Muggle world, something that has been somewhat overlooked in his life in the wizarding world.
One reason could be that he's tired of the constant danger and battles. After all he's been through, he might just want a normal, peaceful life away from magic. Maybe he wants to start a family in the Muggle world where his kids can grow up without the threat of dark wizards.