Easy. When the new NC-17 rating came out in 1990; the religious nutters like Jerry Falwell put pressure on all the major chains to refuse to show NC-17. So of course they got their way and now it's virtually impossible to show NC-17 movies. It wouldn't be hard to keep children out of NC-17 movies, any movie theater could do it, so it's mostly the right wing attacking that which they can't understand and don't like.