I think this is the highest attach rate any title has at the moment. It's likely that it will go down a little by the end of the Wii U's life, but it should still stay over 50% attach rate. That's quite massive. Mario Kart Wii had an attach rate of 35%, Double Dash 32% and Mario Kart 64 of 30%. Super Mario Kart had a significantly lower attach rate, but that's largely because that's before Mario Kart was Nintendo's arguably biggest franchise. Super Mario World, back then, had the highest attach rate of roughly 42%
We're seeing the same trend on the 3DS, with MK7 at 21% attach rate, while MKDS had 15% and the GBA one at 6.7%.
Partially, it's down to Nintendo's games having growing attachment rates all around, but it's more than that down to Mario Kart having become the heart of what Nintendo now is.
While Nintendo of the past (NES-N64) was arguably the greatest developer at creating challenging and large-scale games, they're not that anymore. Or at least, they don't hold the title with the same power, due to there being so many other excellent hard or large games. Today's Nintendo is rather the best at having games that are fun together, with 7 of the Wii U's top 10 sellers being games that are far more fun when there's somebody else in the same room (MK8, NSMBU, Smash, Nintendo Land, 3D World, NSMLU, Wii Party U, 2 titles being online games (Splatoon, Super Mario Maker), and only one large scale, single-player game (Wind Waker).
So in a sense, it shouldn't be surprising that MK8 is doing as well as it is, because if you're buying a Wii U, then you're very likely to be interested in games that are about having fun together. And if you're interested in that, virtually nothing tops Mario Kart.
