Because distributing styles based on other peoples intellectual property is dangerous especially if you live in a country that provides harsh penalties for violation of someones right to their property. Also even if a company allows you to use their IP for a fan site...I hardly doubt they would allow someone to redistribute it let alone sell it.
If you want a site dedicated to a tv show you probably need custom work as in someone makes you exactly what you want for you only and not to be otherwise distributed and then it is your responsibility that your use complies with the owning companies policies on IP. Another thing is while fair use in many cases allows you to build a site for an array of reasons...I don't think that extends to building a style based on someone else's IP and still publicly distributing it even under fair use practices. I believe unless you are buying a fan site that is already done (or something like that) or you hire someone to customize a style for you on your site that the grounds around such a theme are sticky.
Example...I spent over half a year planning and building a site (mostly a chit-ton of back end tools ) and I basically got a cold shoulder when it came to a company which I will leave nameless for now giving me an OK to use their IP in the way I intended and that was me making it for myself, if I was doing this for someone else I would either probably be out of pay or in some serious trouble if I had already built to order and delivered it to them and got paid.
Either way...now I wasted time and am completely screwed.
Best advice...if you want something like this...then hire someone.