It could also be related to Izuku's own self - discovery and confidence building. As he becomes more sure of himself as a hero, he might feel the need to break away from some of the old, almost subservient - like ways of interacting with Bakugou. By not using 'kacchan' anymore, he is asserting his own identity and equality in their relationship.
Maybe there has been a significant event or argument between them. For example, if Bakugou does something that really hurts Izuku's feelings deeply, Izuku might decide to stop using the affectionate nickname as a sign of his disappointment or to create some distance. It could be a turning point in their relationship where Izuku re - evaluates how he addresses Bakugou.
One possible reason could be that Izuku has grown up and wants to have a more formal relationship with Bakugou. As they progress in their hero training and development, Izuku might feel that the childhood nickname 'kacchan' no longer suits their new, more mature dynamic.
Bakugou might be initially shocked. He's used to Izuku calling him 'kacchan' for so long that it's become a normal part of their relationship. So, when Izuku stops, it could throw him off balance.
One possible reason could be that Izuku realizes Bakugou's unchanging and hostile attitude. Despite his efforts to be friends or get along, if Bakugou continuously rejects him or is overly aggressive, Izuku might feel it's a lost cause.
It could be due to Bakugou's public shaming of Izuku. If he embarrasses Izuku in front of others too often, Izuku will reach his limit and snap.
One reason could be that Ash has achieved all his major goals as a Pokémon trainer. He might have won all the important championships and feels there's nothing more to prove in that area. Another reason could be that he has had some kind of traumatic experience related to Pokémon training, like losing a very dear Pokémon in a battle that makes him not want to continue. Maybe he has also found a new passion, such as Pokémon research. He could have met some inspiring Pokémon researchers and decided to shift his focus from training Pokémon to studying them.
If Izuku stops holding back in fanfiction, he might achieve his goals much more quickly. He could show his true power and skills without restraint. For example, in battle scenarios, he could defeat his opponents with ease and surprise everyone around him.
One possible plot could be that Bakugou constantly bullies Izuku, pushing him to his limits in the fanfiction. Maybe Bakugou publicly humiliates Izuku over and over again about his lack of a Quirk at the beginning. This could lead to Izuku finally snapping and standing up to Bakugou in a very unexpected and powerful way, perhaps even revealing a hidden strength or power that no one knew he had.
One possible reason could be that the fanfiction author wants to focus on other characters. Maybe they feel they've explored Naruto's character enough and want to give more spotlight to characters like Sasuke or Sakura. Another reason might be that the plot of the fanfiction takes a turn where Naruto's acting skills are no longer relevant or needed.
I'm not sure specifically as there could be many different takes on it. It might be a fanfiction where the character Izuku reaches a point of apathy or stops caring about things he previously held dear, like his dreams of becoming a hero or his relationships. It could be due to some traumatic event or a slow build - up of disappointments in the story's universe.
It could be due to a change in the quality of the fanfiction available. If the stories started to become too low - quality, with poor writing, inconsistent characterizations, or uninteresting plots, she might have decided to stop chasing them. For example, if the fanfictions were constantly full of plot holes and didn't respect the original Sonic lore, it would be a turn - off.
One reason could be Sasuke's continued pursuit of revenge. If in the fanfiction, Sasuke is so consumed by his need for revenge that he neglects Sakura and their relationship, it might cause her to stop loving him. Another reason could be that Sakura discovers a side of Sasuke she doesn't like, like a cruel streak.