One possible reason could be that Naruto has faced too many betrayals. Maybe his friends turned on him in the fanfic, which made him lose faith and stop caring.
Perhaps in this fanfic, Naruto discovers some dark secrets about the ninja world. These secrets could be so disturbing that they shatter his positive view of the world. For instance, if he finds out that the village elders have been using him or his friends for some immoral purposes all along, it could lead him to stop caring about the village and its values.
One possible reason could be that Naruto has faced so many betrayals and hardships in the fanfiction. For example, if his friends turned on him for some contrived reasons, it might lead him to stop caring. Another reason could be the constant failure in achieving his goals, like protecting the village but always being undermined by political intrigue within the fan - made story.
One possible reason could be that Naruto realizes Sakura's unrequited love for Sasuke. He might feel tired of always being in a one - sided emotional situation with her. Another reason could be that Naruto has his own goals and growth to focus on. His journey to become Hokage and protect the village requires all his attention, leaving little room for his previous feelings for Sakura.
One possible reason could be that Naruto has finally realized that Sakura's heart has always been with Sasuke. After years of chasing after her and getting constantly rejected or overlooked, he just gets tired and decides to move on. He might start to focus more on his own personal growth and the other relationships in his life that are more reciprocal.
One possible reason could be that he has been through so much in his original adventures that he's just tired. He might feel like he's constantly being put in new, often contrived, situations in the fanfiction and he simply wants to step away from all the chaos and drama.
Another reason could be that Hinata herself undergoes a significant transformation. Maybe she has a spiritual awakening or a change in her life goals that make her realize that her love for Naruto was more of an infatuation based on her younger self's ideals, and as she grows, she no longer feels the same way.
One reason could be some sort of betrayal he feels. Maybe he discovers a secret that makes him think Team 7 was never truly his team. For example, if he found out that the village leaders had some ulterior motives for forming the team that involved using him.
Maybe Harry has faced so many traumas and disappointments in the wizarding world that he just decides to distance himself from all the chaos. For example, he might be tired of the constant battles against the Dark Arts and the politics within Hogwarts and the Ministry. He could stop caring about his reputation as 'the Chosen One' and simply want to live a quiet life, away from the spotlight and the expectations.
He might become more withdrawn. Instead of being the energetic and enthusiastic Naruto we know, he could start to isolate himself from others.
It could also be due to a sense of betrayal. If he found out some dark secrets about his family, like his father being involved in something bad that he couldn't forgive. This could make him turn his back on his family and not care about them at all.
One possible reason could be that the author wants to focus on other characters. Maybe they feel that Naruto's story has been told enough in the fanfic and it's time to give other characters the spotlight.