Why is well written fan-fiction so hard to come by?

Suggestions please!
Original fanfiction is welcome, but I would like it to include a well-thought-out magic system.

Fanfiction set in the following universes is also good.
Literature

- Wheel of Time
- Any Trudi Canavan stuff
- His Dark Materials
- Sword of Truth
- Any Brandon Sanderson stuff
- Harry Potter

Anime
- Card Captor Sakura
- Magical Girl Lyrical Nanoha
- Death Note 

Ending with a question so you can submit easily?