Because it’s self-aware. It’s not like any other fantasy about another “chosen one”. Rowell is perfectly aware how cliché and overused these tropes are, so she takes that concept and flips it on its head. Some people think it’s derivative (of Harry Potter in particular). But that’s the whole point- it knows exactly where it stands in the genre and takes full advantage of that. I haven’t read Fan Girl, but I can see how it plays into the idea of fanfiction by transforming an already well-known concept. It’s so meta. Rainbow Rowell really knows her craft.