In such a fanfic, it could be that Percy is betrayed because he represents a threat to the status quo. His growing power and influence might make others feel insecure. They might fear that he will change the way things are in their world. And since Annabeth is the only one who stands by him, it could be that she sees something in Percy that the others don't. Maybe she understands his true intentions better than anyone else, which is why she doesn't betray him.
One possible reason could be jealousy. Maybe Percy has some special abilities or achievements that make others envious. For example, his connection to the sea could be seen as a huge advantage that others might resent. Another reason could be misinformation. Someone might spread false rumors about Percy, leading others to believe he's done something wrong when he hasn't. It could also be that there's a misunderstanding about his relationship with Annabeth. Some might think he's getting special treatment from her or the other way around, which causes them to turn against him.
Perhaps it's because of some external influence. In the world of fanfics, there could be a powerful entity or force that wants to isolate Percy. This entity might manipulate the other characters into betraying him. Also, Percy's own hero complex could play a part. He might be so focused on doing the right thing that he accidentally steps on some toes. And his loyalty to Annabeth could be misconstrued as being part of a clique, causing others to feel left out and thus betray him.