I think the better question is "why would anyone expect TMP to get a PG rating?"
Well, you've got a horrific transporter accident, and you've got a crewwoman from a race so sexually overpowering that she had to take an oath of celibacy just to be allowed to serve with humans. And you've got a robot duplicate of said crewwoman who initially appears buck naked in the shower.
And let's not forget, the Director's Edition is
rated PG. It was always kind of a borderline case.