Money.

If, say, NBC decided to air some show depicting graphic sex/violence/language, etc. you would have millions of interest groups calling for an advertising boycott (think Dr. Laura). Advertisers would get jumpy and they would not advertise on the show, the network would make no money, so despite the ratings bonanza such a show would garner, the network would not show a profit, they would probably lose money on it. Lots. Of course, the networks lose tons of money on the NFL contract (not an assumption, they have said so themselves), but people watch the NFL religiously, so you can promo other shows during NFL programming. So then I just debunked my own theory. Blast, I hate it when that happens.