In the fanfiction, it might be that Sakura abandons Naruto because of unrequited love. She could have been in love with him for a long time, but seeing that he has feelings for someone else or is not able to reciprocate her feelings in the way she wants, she decides to move on and abandon their relationship in the story.
One possible reason could be that Sakura realizes she has different goals or values that are not compatible with being around Naruto. Maybe she discovers a new path in life that doesn't involve him, such as focusing on her own medical ninja career without Naruto's chaotic lifestyle interfering.
Another reason could be related to some external threats or events. For example, there could be a situation where Sakura believes that being around Naruto puts her or others in danger. Perhaps there is a powerful enemy who specifically targets Naruto and those close to him, and Sakura, in an attempt to protect herself or others, chooses to abandon Naruto in the fanfiction.
Maybe she has some unresolved issues from her past, like her unrequited love for Sasuke that still lingers in the back of her mind. This could make her question her relationship with Naruto and lead to her leaving.
Perhaps Kakashi feels that Naruto is becoming too reliant on him and not developing his own skills independently. So, in order to force Naruto to become stronger on his own, he decides to abandon him. This is a tough - love approach. Naruto has great potential, but he needs to learn to stand on his own two feet, and Kakashi may think this is the best way to make that happen.
In some fanfictions, Naruto might abandon his family due to a false prophecy that convinces him he's a danger to them. He may think that by leaving, he can protect them from harm that might come his way because of his past or enemies.
One possible reason could be some sort of dark influence or curse that makes her lose her maternal instincts. Maybe a powerful enemy targets her and in order to protect Naruto from further danger related to her, she feels forced to abandon him. Another reason might be that she is made to believe false information about Naruto, such as that he will bring great disaster to the village in the future.
One possible reason could be that Naruto discovers some dark secrets about the village leadership that go against his values. For example, if he finds out that they have been using underhanded means to maintain power.
One reason could be that Naruto has always been more attracted to those who understand his outcast past better. Sakura doesn't really have that kind of connection with him as she comes from a more normal background. So there's a lack of that fundamental understanding between them which hinders any romantic feelings.
In some fanfictions, it could be due to unrequited love. If Naruto has deep feelings for Sakura but she continuously rejects him in a cruel or thoughtless way, Naruto might feel angry enough to seek revenge. For example, she could publicly humiliate him for his feelings, which would hurt Naruto deeply and lead to thoughts of revenge. Another reason could be a misunderstanding. Suppose Sakura is misjudged by Naruto as having done something terrible when in fact she was innocent. But Naruto, in his ignorance, decides to take revenge. This can create a lot of drama and conflict in the fanfiction.
One reason could be jealousy. If in the fanfic, Sakura gets closer to someone else Naruto cares about, like Sasuke in a different way than he expected, it might make him hate her. For example, if Sakura teams up with Sasuke against Naruto's wishes in the fan - created plot.
One possible reason could be his sense of duty. Sasuke has always been driven by his goals for the village or his personal vendettas. Maybe he feels that his mission is more important than staying with Sakura and their unborn child, believing that his actions are for a greater good that will ultimately protect them as well.
In some fanfiction scenarios, Kurama might abandon Naruto if he believes that Naruto is becoming too corrupted by power. Kurama has a sense of honor in a way, and if Naruto starts to use his power in a way that Kurama doesn't approve of, like becoming overly aggressive or cruel, Kurama could decide to leave. For example, if Naruto starts to use the Nine - Tails' power to dominate others rather than protect them.