Here’s what Joan Graves had to say when asked that very question :
You asked earlier about misunderstandings. I think this is the greatest one. We give far more initial ‘NC-17’s for violence than we ever do for sex. But what happens is, those films don’t go to the press. They just edit it until they get to the top end of ‘R.’ Anybody who gets an ‘NC-17’ for sex, though, goes immediately to the press because they love the publicity. Somehow it’s not as sexy to get it for violence. I know it’s a matter of economics. Filmmakers don’t want to limit their audiences in terms of the number of people that can buy tickets. But I’d like to see nothing more than an appealing director make a very good film that goes out with an ‘NC-17’ like Ang Lee did with Lust, Caution. Then again, that was a foreign language film, so it didn’t appeal to as many people as an English language film would.