Perhaps she has deep emotional bonds with him. It could be that she loves him and is afraid of losing him forever. In many fanfictions, emotional attachments drive characters' actions. So, her not wanting him to escape is her way of holding on to that relationship.
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.
It's also possible that he is a mysterious character who has some sort of connection to Sakura's past or her powers. His presence might trigger something in Sakura, and she can't let him go until she figures out that connection.
Well, it might be a fan - made story within a particular fandom. Maybe 'him' is a character who has done something wrong but Sakura, for some reason like compassion or a deeper understanding of the situation, decides to let him escape. It could involve elements of adventure, mystery, or even some sort of emotional turmoil depending on the overall context of the fanfiction.
Since I haven't read it, I can't be certain, but from the title it seems Sakura and 'him' are likely the main characters. Sakura is the one making the decision to let 'him' escape.
Sakura might have a hidden jutsu or power that she has not yet fully explored. In this dire situation, she could tap into this unknown ability to break free from the snakes. For example, she could have some latent chakra - based power that she discovers can repel or dissolve the snakes binding her.
One possible reason could be that Sasuke is trying to protect Sakura in his own twisted way. He might think that by making her his wife, he can keep her safe from the dangerous world they live in. Another reason could be related to his family. He might see marriage to Sakura as a way to rebuild his family name or something similar. However, his method of forcing her is completely wrong.
Well, in the fanfiction context, Naruto may not love Sakura because he has unrequited feelings for someone else. Maybe he has a deep connection with another character that makes him view Sakura just as a teammate. Also, their personalities might not be as compatible in a romantic sense as one would initially think. Naruto is often impulsive and headstrong, while Sakura can be a bit more reserved in some ways, and this could lead to a lack of romantic attraction.
One possible reason could be that Sasuke is frustrated with Sakura constantly trying to get close to him when he's focused on his revenge. He might yell at her to keep her away and not interfere with his plans.
One possible reason could be Sasuke's continuous absence. He is often away on missions, leaving Sakura feeling lonely and neglected. In fanfiction, this could be exaggerated to a point where she can't take it anymore.
Maybe it's because of Naruto. If Hinata has a closer relationship with Naruto than Sakura does in the fanfiction, Sakura could be jealous. After all, Sakura has her own feelings for Naruto.
One possible reason could be that she might have underestimated Naruto in the past. She often belittled his abilities and dreams, but as she grew and realized his true strength and determination, she felt the need to apologize for her previous behavior.