Well, one reason could be that some fanfics lack originality and just rehash familiar tropes. Another could be that the character development is weak or inconsistent.
Maybe it's because they don't capture the excitement or complexity of the story. Sometimes they're just too brief and fail to give a real sense of what's great about the novel.
That's a broad generalization. Fan fiction quality varies. Some might have weak plots or poor character development, but there are many well-written ones too. It often comes down to the effort and passion of the author.