The thing about reading fanfic (and original slash fic) is that you get used to that particular writing/reading culture after a while. You get used to the frank discussions of sexuality and kink, the close attention to diversity and social justice issues in the text, the unrestrained creativity when it comes to plot. The most amazing, creative, engaging stories I’ve ever read have almost all been fanfiction, and I think part of that is because there’s no limitations placed on the authors. They’re writing purely out of joy and love for the world and its characters, with no concerns about selling the finished product. The only limit is their imagination.
Next to that, most mainstream fiction starts tasting like Wonder Bread, you know?