Maybe for the shock value. It's not the typical view of their relationship, so it can attract readers who are tired of the same old storylines.
Some writers might want to explore alternative character developments. They could think that Sakura has been through so much because of Sasuke that it could lead to hatred. For example, his long absences and the pain he's caused her could be a starting point for this kind of story. It gives a new perspective on the characters and their relationship, different from what we see in the original manga or anime.
Some might write it to explore a different side of the characters. Maybe they want to see how the story would change if there was conflict between Hermione and Harry instead of their usual friendship.
Someone might write it to explore different character dynamics. Kakashi and Sakura have a certain relationship in the original Naruto series, and this gives the writer a chance to twist it. Maybe they want to create a more suspenseful or romantic story.
To explore different character arcs. Maybe they think that if Harry hated Ron, it would force both characters to grow in different ways. It could be a way of looking at how their personalities would develop without their friendship. Also, it could be a way to introduce new plotlines and conflicts in the Harry Potter universe.
Sakura's ability to forgive and still care for Sasuke despite his past actions could also make him proud. In many fanfictions, she represents unconditional love. This selfless love and her ability to see the good in him, even when he was at his worst, could make Sasuke proud of her as a person.
I'm not sure. Fanfiction can be very subjective. Some might find it interesting as it offers a different take on the characters' relationship, but it also goes against the more common portrayal of their bond in the original series.
Maybe Sasuke sees Sakura's growth as a kunoichi. She has become strong and independent, and he admires that. So he wants her to be his.
Well, perhaps the writers feel that the Sasuke - Sakura relationship in the original was a bit forced. So they create fanfiction where Sakura doesn't love Sasuke to explore different relationship dynamics and character arcs for Sakura. For example, she could have a more platonic or even antagonistic relationship with Sasuke, which would be a fresh take on their characters.
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.
One reason could be that in some fanfiction, the characterizations of Sasuke and Sakura might be off. Maybe Sasuke is portrayed as overly cruel to Sakura in a way that doesn't seem true to his character in the original series. Another reason could be that the storylines in these fanfictions are too clichéd, like always having Sasuke constantly reject Sakura in the same uninteresting ways.
In lemon fanfiction, the 'sakura hates sasuke' scenario can stem from various factors. One possibility is that Sasuke's past actions, such as his association with dark forces or his disregard for the safety of the village which Sakura holds dear, could make her resent him. Additionally, if in the fanfiction's world, Sasuke has made decisions that directly oppose Sakura's efforts to build a relationship or a peaceful environment, it would be understandable for her to hate him. For instance, if he undermines her medical work in the village or her attempts to bring the shinobi world together.