I believe the reason is that fan fiction is sort of recognized as "less official". Many people can write a fan fiction, not many people can draw well. Also, in my personal opinion illiterate people have almost ruined fan fictions for me. I just can't stand people's bad grammar. It would be nice for people to get more into fan fictions, but I personally wouldn't buy one unless it was exceptionally written and was similar to book length. But this is the reason people don't buy fan fictions: with art, you see the picture, you like it, you buy it. With fanfics, you don't really know what you are buying, the quality of it. You can't read it and then decide because, well, then you have already read it. I hope that fan fictions gain more popularity, but I don't see anybody buying them any time soon...