1. The importance of heavy, heavy marketing.
2. Certain franchises are central to the Nintendo audience and will always be bought; others are more "flash in the pan"
3. Mario Kart is by far Nintendo's most perennial franchise.
I'm not sure I agree with the "Mario Kart is a huge step up over previous entries where as everything else was not" argument. It may well be the reason. But to me, adding anti-gravity doesn't make the game seem any less iterative. Which isn't a bad thing, by the way. It's just from an outside point of view, the draw of something like SM3DW being a step up but not massively over SM3DL is the same thing as MK7 -> MK8. I don't see the distinction.
Certainly in the post above, he's saying that adding local multi-player to NSMB -> NSMBWii was "something completely new", but then doing the same for SM3DL -> SM3DW isn't.