Skyrim is open to a wider range of audience then Witcher or the Fallout's are. Skyrim has many teen players and advlt players, so if they added in six and nudity it may hurt sales from the teen category. Their parents may not want them to purchase a game that has such content. Oblivion and Morrowind both did not have six or nudity included.
That was left open for the modders to add in, and they sure did that. six has already been added in, you can find it on the Nexus, it is fully animated and everything, there are nvde textures to go along with it.
Fallout is post-apocalyptic game, where greed, murder, and sins run wild. Plus New Vegas took place on the Strip, so that kind of had to be included. What's Vegas without strippers?

And I don't remember Fallout 3 really having any type of six, other than small sixual themes in it.
The Witcher is based off a series of advlt/mature books, very advlt books. six is part of your Geralt, he just is always getting it on. It's also a part of the "Witcher world", the game wouldn't be the same if you weren't bangin' people all the time

Skyrim is set in a harsh world, wear a dragons are constantly attacking small villages, cities, and people. As the Dragonborn, it's your duty to stop that, you don't really have time to stop and have six, maybe after the main quest it would work, but not with dragon's ruling the sky. I could also see the Temple of Dibella serving as some sort of brothel, but again, Skyrim has a larger age groups/audience that plays the game.
Say you were 10-14 again, you really wanted to get this game, but there was just some nudity and six in it, would you parents let you get it if they saw that that was in the game? Probably not..so that would help mess up sales.
So if you really, really want six in your Skyrim, just go get the six mods.