One possible reason could be a misunderstanding. Maybe there was a miscommunication between them, like Ginny thought Harry was still too preoccupied with his past traumas and not fully committed to their relationship. Another reason might be external pressure. In some fanfictions, there could be outside forces, such as dark wizards threatening Ginny and she leaves Harry to protect him.
One possible reason could be miscommunication. Maybe they had a big fight over how to raise their child and Ginny, feeling unheard, decided to leave. Another reason might be external pressure. For example, if some dark forces were targeting their family again and Ginny thought leaving would keep Harry and the child safer. Or perhaps Ginny had some personal issues she couldn't deal with while being in the family, like feeling she lost her own identity in the relationship.
One possible reason could be that Harry realizes he has stronger emotional and intellectual compatibility with Ginny. He might find that Ginny understands his complex life as a wizard better, especially with her own experiences in the wizarding world. Also, Ginny is more in tune with Harry's adventurous spirit, which Cho might not be as much.
One reason could be that he discovers a new threat and decides he can't stay with the Dursleys any longer for their safety. Another might be that he simply can't bear their mistreatment anymore and finds an opportunity to escape.
One reason could be to escape the constant scrutiny and expectations that come with being 'The Boy Who Lived' in Magical Britain. He might feel suffocated by the fame and the never - ending battles against the Dark Arts in his homeland. Another reason could be a quest for new magical knowledge. Magical Britain has its own set of magical teachings, but there could be other, more esoteric forms of magic in other parts of the world that he wants to explore. Also, he might be running away from a personal tragedy or a broken relationship. Maybe a falling - out with his friends or a loss of someone dear to him makes him want to start anew elsewhere.
One possible reason could be that the fanfiction deviated too much from the original character traits of Hermione. Fanfiction often takes creative liberties, and if it strays far from what makes Hermione who she is in the original series, it might make her seem out of character, leading to a decision to leave.
Another reason might be some kind of external influence, like a powerful dark magic that clouds Harry's judgment. However, this is often a stretch from the original concept of the characters. Fanfiction writers sometimes like to explore extreme scenarios to add drama to the story.
One possible reason could be jealousy. Maybe Ginny gets jealous of Harry's friendship with Hermione. Another reason might be differences in their future plans. For example, Harry may want to focus on his Auror career more than Ginny anticipates, causing a rift between them.
One reason could be that the official Harry Potter canon is so extensive and detailed that it overshadows fanfiction. The original story has such a rich world, characters, and plotlines that it becomes the main focus for many fans, leaving less room for fan - created stories.
One possible reason could be miscommunication. Maybe Ginny feels neglected by Harry because he's so caught up in his own heroic deeds and doesn't pay enough attention to her. So she might seek comfort or excitement elsewhere.
One reason could be that the fanfiction is set in a different universe where the characters' personalities are slightly altered. So, this 'new' Harry just doesn't click with Ginny in that version. For instance, if the fanfic is a darker take on the story, Harry might be more jaded and not open to a relationship with Ginny.