My thoughts exactly "why not" . I mean most games comming out on the PS3 are Motion Controller compatible and if Skyrim is compatible you'll have at least the option to use the motion controller and if you don't want to use the MC's then you don't have to. Again it's just an option.
Because this isn't a PS3 exclusive, it's a multiplatform, if PS3 got Move support, 360 players would want Kinect and I and every other PC gamer would rage. Plus, it's too much work, adapting the gameplay to Move and then to Kinect is hard and would take too much precious time from the developers, that could be used to make things like quests, NPCs, locations, etc.
Also, to make a game support Motion gaming, the game would either have to be simple so it can be implemented later through patches, or build it from the ground with Motion gaming on mind. And both cases are not happening with Skyrim, and won't.