All of the Networks still rely WAY too much on Nielsen ratings and don't take into account the numbers from sites like Hulu or views of episodes on their own sites. It's an antiquated system and because of it good shows are cancelled. As to what makes them air a show like Napoleon Dynamite, well I'm with Nik on the knife wielding blindfolded network exec.
There are a few instances of internet campaigns to save shows that worked (see: Chuck) but for the most part, if they think they can take the Nielsen numbers to the ad execs and get the most green then that is what is going to happen.
BTW, Does anyone here know a Nielen family? I don't.