Well, Mario Kart drives hardware and hardware drives Mario Kart because it's a "get with the system game",
hardware base is the bigger driving factor for Mario Kart of course, but there are 34.000.000 people that played it, so the awareness for the franchise is there.
3D World was released in a timeframe with NSMBU, NSMBL and 3D Land, so it was a bit "polluted" with Mario platformers.
Mario Kart 7 is already at 8.080.000 units (March 2013, so it should be several 100.000s more by now).
You can argue that 3D Land has 8.290.000 units, but I'd still say that Mario Kart 8 is the bigger game than 3D World.
Yeah, that's realistic considering the current situation, we'll see how things evolve.