It could be due to outside influence. For example, an enemy of Sakura might have bribed or threatened her friends. Her friends, being weak - willed, then choose to betray her.
It's also possible that her adventure - team friends betray her. In a fictional world, if there are some high - stakes situations like a quest for a powerful artifact, they could betray her for their own gain, such as getting the artifact all for themselves.
Perhaps there was a misunderstanding. Maybe Sakura said or did something that they misinterpreted, and instead of talking it out, they chose to abandon her.
Maybe they were jealous of Bell's success or popularity in the story.
One possible reason could be jealousy. Maybe Ash has achieved more in his Pokemon journey than his friends, and they can't handle it. Another reason might be some misunderstanding. For example, an enemy could trick Ash's friends into thinking Ash has done something bad. Or perhaps there's some sort of power struggle. If there's a powerful item or a rare Pokemon involved, and Ash is the main contender, his friends might be tempted to betray him for their own gain.
In some fanfictions, Ginny might be written to betray Harry because of a prophecy. The prophecy could mislead her into thinking that Harry was going to bring doom, and so she decides to act against him to prevent that. It's a different way of looking at their relationship compared to the original story, where their bond was based on love and trust.
One reason could be a power struggle. If there was a rare and powerful Pokémon at stake, Ash might be tempted to betray Korrina to get it for himself. In the Pokémon world, power and rare Pokémon are highly coveted.
Power could be a reason. If there's a chance for Ron and Hermione to gain a great deal of power by turning against Harry, some fanfictions might explore this. For instance, a new dark power might offer them something they've always desired, like the ability to reform the wizarding world in their own image. They might think Harry would oppose this, so they betray him to get what they want.
One possible reason could be jealousy. If Harry gets more attention or power than the Weasleys, they might feel envious and turn against him.
Sakura might be waiting for the right moment. In the fanfiction, perhaps she wants to tell him when they are in a more peaceful and stable situation. She could also be concerned about the safety of the baby. If there are threats around, she might not want to make the baby a target by making the news public, even to Sasuke.
One possible reason could be that she sacrifices herself to save Naruto from a powerful enemy. In the heat of battle, she might put herself in harm's way to ensure Naruto's survival.