How could you ever think otherwise?
Probably something irrelevant and inconsequential like differing opinions, perhaps.
I'm not particularly surprised. Nintendo had to release something eventually that would start seriously gaining momentum, and it's also not particularly surprising that Mario Kart was that something. I of course hope they can maintain this momentum with a string of quality releases throughout the year, and with Hyrule Warriors, Bayonetta 2, 'X' and Super Smash Bros. 4 in the pipeline, that may very well be the case.