Maybe they're leaving because they want a fresh start away from the memories and pressures in England. After all the battles and events that took place there, they might feel the need to get away and build a new life in a different place, free from the constant reminders of the past.
It could also be related to Ginny's career. If she has an opportunity to play Quidditch for an international team, they might decide to move to the country where the team is based. Harry would support her wholeheartedly and go with her. This would give them a chance to experience a new country and its wizarding community while Ginny pursues her passion.
One reason could be to explore the wizarding world outside of England. There are many magical places they might be interested in, like the wizarding communities in France or America. They could be on a mission to learn more about different magical cultures and bring back new knowledge to England.
One possible reason could be to escape the danger and chaos that might be brewing in England within the wizarding world. Maybe there are dark forces targeting them specifically, and leaving is their only option for safety.
One reason could be to escape the trauma of the war. They've been through so much in England, and leaving gives them a chance to heal in a new environment.
Maybe he's tired of the spotlight in England. In some fanfictions, he could be seeking a new start away from the prying eyes and the constant media attention that comes with being in England.
One reason could be to escape the danger and dark forces that still lurk in England even after the main battle. They might feel that leaving is the only way to start anew and be truly safe. Maybe they want to explore other magical cultures around the world and learn more about magic that is different from what they've known in England.
One possible reason could be a misunderstanding. Maybe Ginny misinterprets something Harry does and feels she can't stay in the relationship. Another reason might be that she feels overshadowed by Harry's fame and the pressure of being part of the Potter family. It could also be that she has some personal goals or dreams that she realizes she can't pursue while being with Harry and taking care of the kids.
It could be due to a threat to their lives within the village. There might be a hidden group in Konoha that has a vendetta against them. Naruto and Tenten, not wanting to put their friends and family in danger, choose to leave. They might plan to return once they have found a way to neutralize the threat.
One possible reason could be to escape the danger in England. Given the dark forces that were at play, especially Voldemort's rise, they might have felt it was safer to leave. Another reason could be for a fresh start. Maybe they wanted to explore new magical cultures and communities in other parts of the world.
They might go to France. France has a rich wizarding history and a beautiful wizarding community. The Beauxbatons Academy is there, and they could explore the magic related to it. There are also magical places like the French Ministry of Magic which would be interesting for them to visit.
In the fanfiction world, it could be a prophecy. There might be a prophecy that says Harry's presence in Ginny's life will bring great harm to her or the wizarding world. So, for the greater good, Harry leaves. He believes that by sacrificing his relationship with Ginny, he can save her and everyone else. It's a very noble but also a very heart - breaking decision.
One possible reason could be to protect Teddy. Maybe there are some threats in England that Harry wants to keep Teddy away from. For example, there could be some dark wizards or dangerous magical creatures that target Teddy for some reason.