One reason could be that in some fanfictions, Naruto might feel that Sakura is too obsessed with Sasuke and neglects their friendship. He values loyalty and may see her one - sided love for Sasuke as a form of betrayal to their team bond.
One possible reason could be for the sake of creating a more complex love triangle in fanfiction. Maybe the writer wants to explore a different dynamic where Naruto and Hinata's relationship is strengthened by having a common dislike for Sakura. It could also be that the fanfiction writer is trying to give Hinata more agency and make her a stronger character by having her oppose Sakura in some way, perhaps in matters related to Naruto's affections.
Sometimes, fanfiction authors want to explore the reactions of other characters towards such a situation. How would Ruby, Weiss, Blake, and Yang react if everyone hated Jaune? It can also be a way to introduce new conflicts and alliances within the RWBY group. If everyone hates Jaune, who will side with him? Who will be against him? This creates a complex web of relationships that can be interesting to write about.
In fanfiction, the evidence often lies in the way the author portrays Naruto's interactions. For example, Naruto may be shown being cold or dismissive towards Sakura, like avoiding her company or not showing any romantic interest. Meanwhile, with Hinata, he could be depicted as being more affectionate, like blushing around her or always coming to her aid. But it's important to note that this is all in the realm of fan - created interpretations and not the official canon.
One possible reason could be that in some fanfiction, Naruto might be written to hate them because of their early treatment of him. Sakura was often dismissive of Naruto in the beginning, and Ino was more focused on Sasuke too. So in a different narrative, Naruto could hold grudges against them for not seeing his worth initially.
Well, this kind of fanfiction might be an interesting take on the character. It could explore a darker side of Harry Potter that we don't often see in the original series. Maybe the author wants to show how all the pressure and trauma he has endured could lead to a more cynical view of those around him.