Another reason might be related to Naruto's own growth. He might feel that by allowing Sasuke to go, he is also testing his own strength and resolve. He trusts that he will be able to bring Sasuke back in the end, and this is just a part of their journey together. In the context of the fanfiction, there could be some unique plot elements that make Naruto believe this is the best option for both of them at that moment.
Perhaps Naruto realizes that forcing Sasuke to stay would only create more resentment. By letting him escape, he hopes that Sasuke will have time to think and eventually return willingly. In the fanfiction, it could also be that Naruto has some sort of premonition that Sasuke's escape is necessary for a greater good in the future, like saving the village or the ninja world in a different way than if he were held back.
In this fanfiction, the event could lead to a more nuanced relationship. Naruto's act shows his trust in Sasuke, and Sasuke might start to trust Naruto in return, albeit in a very convoluted way. They might communicate in a more unspoken manner. Naruto's letting Sasuke escape could also lead to a period of separation where they both grow independently, and when they meet again, their relationship has evolved. It could be a more mature, less one - sided relationship where they both respect each other's choices more.
It could also be due to a misunderstanding. Perhaps there is some misinformation spread about Sasuke in the fanfiction, like false rumors that he has a hidden and malevolent plan. Naruto, being someone who values their friendship deeply, would be scared at the thought of Sasuke turning into an enemy because of these untrue stories.
Naruto could use her shadow clone jutsu. She could create multiple clones while Sasuke is distracted and then make her escape. Since she has a large amount of chakra, she can afford to create a lot of clones to confuse Sasuke.
One possible reason could be that he overestimated his abilities during a battle in the Wave arc. Maybe he took on an enemy that was much stronger than he thought and got fatally injured. Another reason could be that he sacrificed himself to save Naruto or someone else. It's also possible that there was some sort of betrayal within the group they were with, and Sasuke was caught off - guard and killed.
Another reason might be related to their different ideologies within the ANBU. Naruto may have believed in a more collaborative and friendly approach within the organization, while Sasuke might have been more focused on individual achievement and power. This fundamental difference could lead to Naruto hating Sasuke in the fanfiction.
One possible reason could be to protect Hinata. Maybe Sasuke has unknowingly made a move or decision that Naruto perceives as a threat to Hinata, so he scares Sasuke to make him back off.
One reason could be Sasuke's thirst for power. If he believes Naruto is the only obstacle in his path to ultimate power, he might be driven to kill him. In some fanfictions, Sasuke may be influenced by a dark entity that twists his mind into thinking Naruto is a threat that needs to be eliminated.
One possible reason could be that he has some crucial information that Sakura needs. Maybe he knows secrets about their world or a powerful enemy, and if he escapes, that knowledge will be lost.
In the fanfiction, Naruto could stop chasing Sasuke because he has a moment of self - realization. He might understand that his own identity and purpose in life are not solely defined by bringing Sasuke back. He could find new goals and dreams that don't involve Sasuke. Also, there could be a major event in the ninja world that makes Naruto shift his focus away from Sasuke, like a new and powerful enemy that threatens the entire village or region.
One possible reason could be a major ideological difference that has grown over time. Maybe Sasuke has adopted a more extreme view on protecting the village or avenging his clan that Naruto can't support. Another reason might be a misunderstanding that has festered. For example, if Sasuke was misinformed about Naruto's actions and intentions and has cut off ties because of it.