One possible reason could be that Karin has unique abilities that are more useful to Sasuke in his goals. For example, her sensing abilities might be crucial in his missions. Also, Karin has a deep loyalty towards Sasuke which he might value more than Sakura's feelings at that time.
One reason could be to create drama. The idea of a relationship being broken by cheating is full of emotional turmoil which can make for an interesting read. Another reason might be some writers' preference for the Sasuke - Karin dynamic. They might see potential in their relationship that wasn't explored in the original series.
One reason could be that Sasuke realizes his true feelings for Karin were always there. Maybe in the fanfiction, he had some sort of epiphany that his connection with Sakura was more of a convenience or misunderstanding, while his bond with Karin was deeper on an emotional level.
It could be due to some fans' different interpretations of the characters. Maybe they see a certain chemistry between Sasuke and Karin in the original series that they want to explore further, even if it means creating a scenario where Sasuke cheats on Sakura. Also, fanfiction is a place for creativity and exploring different possibilities, and this is just one of the many extreme or different ideas that can come out of that.
There are fanfics like that. Sasuke might leave Sakura for Karin because of Karin's loyalty. Karin has shown unwavering loyalty to Sasuke throughout the series. She was willing to risk her life for him on multiple occasions. In contrast, Sasuke might think that Sakura's love is more of a childhood crush that she hasn't fully grown out of, and he may be drawn to Karin's more mature and selfless loyalty.
It could be because of a dark power corrupting Sasuke. Maybe an ancient evil force takes over him and makes him see Sakura as an enemy. In some fanfics, this kind of external influence can completely change a character's actions. Also, it might be related to Sasuke's pursuit of power. If he believes Sakura is standing in the way of his ultimate power - seeking goal, he could be driven to kill her.
Maybe to explore different character arcs. The writer might want to see how Sakura would develop as a character if she strayed from her loyalty to Sasuke. It could also be a way to introduce new characters or relationships into the Naruto fanfic world.
One possible reason could be Sasuke's focus on his revenge. His thirst for vengeance against Itachi might make him blind to Sakura's feelings and neglect her as he is constantly preoccupied with getting stronger to achieve his goal of revenge.
One possible reason could be that Sakura has finally grown tired of Sasuke's aloof and cold behavior towards her. She might have realized that she has been constantly chasing after him while getting little in return, so she decides to ignore him as a form of self - preservation.
One possible reason could be his long - standing feelings for Sakura. Despite his journey and time with Karin, his original connection with Sakura might have resurfaced. Sasuke has known Sakura since they were young, and those early memories and emotions could have influenced his decision.
Perhaps Sasuke was testing Sakura's loyalty or strength. He may have thought that if she couldn't withstand his aggression, she wasn't worthy of being in his life. Also, his own inner demons, like the hatred he carried from his past, could have flared up and led him to almost kill her without fully realizing what he was doing.