I guess it is because it is more unexpected. For example: Last year on Trio, I was watching an episode of "I Claudius" - a '70s BBC miniseries on the Roman empire - and in walk three topless dancers. I expect nudity on Showtime, but the shock of nudity on a more "respected" show is exciting. Nudity on MTV would (actually it should) seem reasonable, but nudity in a "religious" docudrama makes you do a double-take. Maybe because the nudity isn't the central focus - stuffy/boring history is? See my "Caveman tv nudity" thread for another example:
https://forum.oneclickchicks.com/showthread.php?t=18429. A showing of the SI Swimsuit Special probably had better ratings, but the "Walking with Caveman" documentary had the nudity.
Strange fetish maybe... don't know, but it's kind of along the same lines as a downblouse or upskirt - you can see more explicitness in an FHM mag, but the candid nature of the whole situation is the exciting part.