If this is true, it's because fanfic writers use characters and settings that were pre-popularized for their convenience. They're writing for an audience that already exists. It's the same reason it's much easier for unknown bands to get people excited about covers of popular songs than their own music.

That said, if you write well, people will want to read what you write whether it's fanfic or not. If you write badly, you need some other way to interest your audience.