Largely for the same reasons that Homer Simpson wouldn't be let anywhere near a real nuclear reactor.
If you want it in business speak, fan productions are to a serious moneymaking business what Pacific Island cargo cults are to the military that won an industrial war in two hemispheres. They are not a good investment. What fan productions have in creativity, they generally lack in professionalism, cost control, and the ability to capture the interest of the general audience.
Fan productions can be successful, true. But this is as rare as a handful of actors walking into the woods with a camcorder and recording a 250 million dollar movie, or a bunch of frat boys dicking around in Halo multiplayer and kicking off a multimillion dollar comedy series. For every raging success you get, you'll also get ten mediocre projects and three or four more that will poison the brand.