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.
There probably is. Fanfiction writers like to take existing characters and create different storylines. But it's important to note that this isn't part of the official Naruto story. In the original, Sasuke and Sakura are a couple, and Karin has her own relationship with Sasuke which is more of comradeship in the official plot. Fanfiction just gives people the freedom to deviate from the original.
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.
There could be. Fanfiction writers sometimes come up with very creative and sometimes controversial ideas. However, this particular plot would be quite a deviation from the original story's themes of loyalty and friendship. In the original, Sasuke has his own complex relationships that don't involve such unfaithful behavior in this way.
Some writers might want to explore different character traits. They may think that Sasuke has a more complex personality that could lead him to make such a mistake. Also, it could be a way to show Sakura's reaction in a difficult situation and how she might grow or change because of it. In addition, it might be an attempt to break the traditional 'happy ending' mold of their relationship in the original story.
It could also be a way for the writer to express their own dissatisfaction with the original relationship between Sakura and Sasuke. Maybe they felt that Sakura's character was not well - developed in the context of that relationship and decided to create a more 'dramatic' storyline through cheating.
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.
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 reason could be to create drama. The Naruto series has a large and passionate fan base, and a plot like this would shock readers and generate a lot of discussion. Another reason might be that the writer wants to explore different aspects of the characters. Maybe they think Sasuke has a side that could be tempted to cheat, or they want to see how Sakura would handle such a situation.
Some fanfic authors might write it as a form of wish - fulfillment. They might prefer the Naruto - Sakura relationship more than the Sasuke - Sakura one. So, they create a scenario where Sakura turns to Naruto in a romantic way. Additionally, it could be a way to explore the characters' personalities further. For example, how Sasuke would handle being betrayed and how Naruto would deal with being involved in such a situation. It gives a chance to dig deeper into their emotional and moral aspects.
It's an interesting but controversial concept. On one hand, it shows the creativity of fanfiction writers to explore different relationship scenarios. But on the other hand, it can be a bit off - putting as it goes against the essence of the characters as we know them in the original series. For example, Sasuke is more focused on his revenge and later redemption, not on such relationship betrayals.