Hmm... I wonder, in addition to userbase and general franchise fatigue, if 3D's numbers could also be partly because, since it's on the 3DS, people simply thought/assumed to be a sequel and a game of the same... quality as the two entries on the DS, and thus were put off from 3D due to how 358/2 Days and Re:Coded turned out, just not wanting to take that risk again. Combine that with the small userbase and franchise fatigue (people beginning to learn that it would probably be a bad idea to actually by a 3DS just for KH3D, because that probably would have been the only KH game it was ever going ot get anyway, which wouldn't have done it any favors) and the numbers start to make sense.