I saw a documentary about censorship in cinema and they talked about this. According to the film, after they came up with the movie rating system, people associated an "X" rating with porn films. When "Last Tango in Paris" came out though, it was fairly successful in the box office, which gave studios some hope. Audiences were still reluctant to watch an "X" film, so they eventually came up with the NC-17 rating to distinguish between cinematic movies and porn. Studios figured the rating would be their saving grace.
The problem was, the first movie to be released with an NC-17 rating was "Showgirls". The film was so pathetically bad, that it left a stigma on the rating and studios didn't want to take the chance on being associated with it. Therefore, they cut out any scenes needed to keep an R rating.
When the PG-13 rating came out though, they found more families were taking their kids to the movies. It was no longer R for adults and G or PG films for the kids, but no real rating to give teenagers the chance to watch a movie without adult supervision. . PG-13 seemed like the happy medium and allowed the widest range of viewers. Since the rating was introduced, PG-13 movies have become some of the highest grossing films, regardless of how good or bad they might be.
So now, studios try to edit their films down just to get that rating. Some Indy film makers have continued making NC-17 rated films, but I'll bet most people can't name 10 of them they've seen; whereas just about anyone can name at least 20 PG-13 films.
So yeah...sadly it's all about the Benjamins.