Maybe in the fanfiction, Sakura's character is depicted in a way that Naruto no longer finds appealing. For example, if she is shown as being too bossy or not true to her original character traits. Another reason could be that Naruto discovers new relationships or goals that make his previous feelings for Sakura seem less important.
It could be that the story arc in the fanfiction takes a turn where Naruto realizes that his feelings for Sakura were more of a childhood infatuation. He might start to see her more as a friend or teammate rather than a love interest. Also, external events in the fanfiction such as a new threat to the village that makes him reevaluate his emotions and focus on more important matters.
One possible reason could be that Naruto's character development leads him to focus more on other aspects like his ninja duties or his bond with other friends. As he grows, his priorities shift.
One reason could be that Naruto has always been more attracted to those who understand his outcast past better. Sakura doesn't really have that kind of connection with him as she comes from a more normal background. So there's a lack of that fundamental understanding between them which hinders any romantic feelings.
One possible reason could be a misunderstanding. Maybe Sakura said something that Naruto misinterpreted as being hurtful or dismissive of his feelings. For example, if she made a comment about his abilities in a negative way during a crucial moment, it could lead to him distancing himself.
One reason could be that the fanfiction changes Kakashi's teaching methods too much. Naruto is used to Kakashi's unique style in the original story, and if it's drastically altered in a way that doesn't seem right to Naruto, he might not like it.
In certain fanfictions, Naruto might have a pre - existing relationship or unrequited love for someone else. This could make him overlook Hinata or even seem to dislike her because his heart is set on another character.
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.
Well, Naruto has always been there for Sasuke in the original story. In fanfiction, Fem Sasuke might secretly like Naruto because she sees his loyalty. He's constantly trying to bring Sasuke back to the right path. This loyalty could translate into a deeper affection in the fan - created stories.
In some fanfictions, it could be due to unrequited love. If Naruto has deep feelings for Sakura but she continuously rejects him in a cruel or thoughtless way, Naruto might feel angry enough to seek revenge. For example, she could publicly humiliate him for his feelings, which would hurt Naruto deeply and lead to thoughts of revenge. Another reason could be a misunderstanding. Suppose Sakura is misjudged by Naruto as having done something terrible when in fact she was innocent. But Naruto, in his ignorance, decides to take revenge. This can create a lot of drama and conflict in the fanfiction.
One reason could be jealousy. If in the fanfic, Sakura gets closer to someone else Naruto cares about, like Sasuke in a different way than he expected, it might make him hate her. For example, if Sakura teams up with Sasuke against Naruto's wishes in the fan - created plot.
Maybe in the fanfiction, Naruto's positive and energetic personality attracts Ino more. He is always so full of life and has a big heart.
One possible reason could be that she sacrifices herself to save Naruto from a powerful enemy. In the heat of battle, she might put herself in harm's way to ensure Naruto's survival.