One reason could be that Harry has had enough of the wizard world's troubles. He might want a normal life away from magic. After all he's been through, like fighting Voldemort and losing many friends, he may just be tired of the whole wizarding business.
Perhaps he refuses because he wants to break free from the expectations that come with being a wizard in the fanfiction world. In the original story, he was always in the spotlight. Here, he might be seeking anonymity and a chance to be his own person without the pressure of magic.
It could also be that he has seen the darker side of wizardry in the fanfiction, such as new forms of dark magic or corrupt wizards. This has made him decide that he doesn't want to be associated with that world anymore. He may want to distance himself from the potential chaos and danger that being a wizard in the fanfiction might bring.
It could also be that Harry Potter in fanfiction is a more self - reflective version of himself. He has already been through so much in the original series that he has grown tired of fighting. He may realize that there are other ways to resolve conflicts, such as through dialogue and understanding, rather than resorting to physical or magical combat as is often depicted in fanfiction.
One possible reason could be fear. Maybe he's heard some scary rumors about Hogwarts, like the Forbidden Forest being full of dangerous creatures. Another reason might be that he doesn't want to leave his normal life behind. He has friends in the Muggle world and he might be reluctant to give that up for a world full of magic that he doesn't fully understand yet.
One possible reason could be that Harry might feel he's not ready for marriage. He has been through so much trauma from fighting Voldemort and might need more time to focus on himself and his own healing. Another reason could be that he might view Ginny more as a sister or a close friend at this point in the fanfiction, rather than a romantic partner.
One possible reason could be that Harry has grown tired of being Dumbledore's pawn. He might feel that he has been constantly pushed into dangerous situations for Dumbledore's grand plans and he just wants to focus on his own life and safety for once.
One possible reason could be that he's tired of being used by the British wizarding world. He has faced so many challenges and been through a great deal of trauma, like fighting Voldemort multiple times. He might feel that it's time for others to take responsibility and that he wants to focus on his own life for a change.
One possible reason could be his fear of losing his independence. Harry has always been a bit of a lone wolf, used to relying on himself. Marriage might seem like a threat to his freedom to do as he pleases.
One possible reason could be that he wants to prove his own strength without relying on a wand. In the wizarding world, wands are seen as a crutch for magic, and Harry might want to show that he can be powerful even without one.
Maybe for the sake of novelty. People might be tired of the traditional good - vs - evil story in the Harry Potter world and want to explore a completely different take on the character. It could also be an attempt to show how power can corrupt, even someone like Harry who was initially on the side of good.
Maybe the writer wants to be different and create a shock factor. By making Harry a dark wizard, it goes against the common perception and grabs attention.
One possible reason could be that he's tired of the constant pressure and spotlight that comes with competing. He has been through so much already and just wants a break from the high - stakes competitions like the Triwizard Tournament. Maybe he's also seen how competition can bring out the worst in people and doesn't want to be part of that.