Not sure, but from what I've understood the MPAA REALLY doesn't like sex in movies. Violence can be as graphic as you imagine it, but when sex on it's own or the two mix...you get an NC-17. The Tree rape probably was the reason why it was given that rating. Had Raimi decided to not actually show the vines and branches entering the skirt, he may not have received the NC-17. BUTTTT, had Raimi not went through with it, the film may not've been as memorable as it's cult classic status.

I'm sure most of you have seen this, but if you really wanna know how bad the MPAA is rent "This Movie is Not Yet Rated". Explains so much, and makes me thankful everyday that Unrated DVD is now an industry norm.