It was rated 'X' in the 70s, and that's been "updated" to NC-17 without much thought. I believe what qualifies it for that is the fact that it's not just full frontal but sustained full frontal that keeps it there.
And I suspect you overestimate the amount of actual nudity on HBO. HBO does a lot of quick-cuts and suggestive pans that show far less than you'd think, and for much less time. Most of what you see on HBO is bare tits and ass, not full frontal.
Then there's also the very direct sexual theme of it, as opposed to the sexuality being incidental.
Not to suggest I think there's anything there that needs to keep older teens away. I think anyone over 15, offhand, isn't likely to be deeply perverted by it in any way, even without adult supervision.
OK--you're right about HBO. Plenty of tits and ass but nothing frontal below the waist. You're probably right about the sustained full frontal nudity and sexual themes keeping it at an NC-17. "Watchmen" had one character (Manhattan) who was nude through almost the entire movie and there was plentry of full frontal...but that was CGI and not in a sexual context.
You clearly haven't seen Game of Thrones if you think there's no frontal nudity on HBO. There's more in this, but the only way this could get re-rated to "R" would be if they PAID to resubmit it to the MPAA for review.
Given the lack of interest in the film (it's a pre-MTV talking heads movie, so it's certainly slow by modern standards, though it's quite well written and done in that context) that's not gonna happen.