One possible reason could be that Sakura realizes Sasuke is on a path she can't follow. Maybe he's too consumed by his revenge and she decides it's better for her own well - being to leave him be. In some fanfictions, she might also find someone else who values her more and she chooses to focus on that new relationship instead of chasing after Sasuke.
In certain fanfictions, Sakura may leave Sasuke alone because she feels that she has tried everything to reach him but has failed. For example, she has continuously offered her support and love, but Sasuke remains cold and distant. So, she decides that it's time to respect his aloofness and move on with her life. This could lead to her exploring new friendships, career opportunities, or other romantic interests within the fanfiction world.
One possible plotline could be that Sakura decides to raise the child on her own. She focuses on her medical ninjutsu to provide for the baby. Another could be that she tries to find Sasuke again to make him take responsibility. Maybe she faces challenges from the village as a single mother, but her friends support her through it all.
One possible plotline could be that they leave to seek a new form of power. Maybe they've discovered a threat that the Hidden Leaf isn't aware of, and they believe they can find a solution outside. Another could be that they're on a journey of self - discovery. Sakura has always been in Sasuke's shadow to some extent, and this could be her chance to truly find herself while being with him. Also, perhaps they leave due to some political unrest in Konoha that they don't want to be a part of.
They might leave to seek new forms of power or knowledge. There could be things outside of Konoha that they believe will help them become better at their ninja skills. For instance, they might have heard of a special dojo or a hidden scroll in another land that could enhance their abilities.
One possible reason could be that Sasuke's past traumas and his continuous search for redemption make him feel unworthy of Sakura's love. He might think he's dragging her down and decide to end the marriage for her own good.
They could be leaving in search of new training opportunities. Konoha might not offer the advanced or specialized training they seek. So, they decide to leave to find a place where they can grow stronger and expand their abilities.
A plotline might involve Sakura facing a lot of judgment from the village. But she stands strong and raises the child with the help of her friends like Naruto and Ino. They support her emotionally and sometimes even materially. As for Sasuke, he might be on a journey of self - discovery and when he hears about the pregnancy, he has to decide whether his path still lies away from Konoha and his family or if he should change his ways.
One possible reason could be Sasuke's continuous absence due to his solo missions. He's always away, leaving Sakura feeling lonely and neglected in their relationship. This lack of emotional and physical presence can gradually erode the bond between them.
One possible reason could be that Sakura has finally grown tired of Sasuke's aloof and cold behavior towards her. She might have realized that she has been constantly chasing after him while getting little in return, so she decides to ignore him as a form of self - preservation.
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.
One reason could be that Sasuke really values Sakura's friendship and is afraid of losing it to someone else. Maybe she sees others getting closer to Sakura and feels left out.