Mario Kart 8: Deluxe is essentially the same version of the WiiU with the same graphics (at least they looked identical to me) but with new content added to it. On MK8-Deluxe there's now the possibility to hold 2 items, proper Battle Stages and new characters to play as. That's just awesome!
April 28th isn't that far away but it would have been great as a launch title for sure.
the basic idea is to not force people to have to decide between zelda and mario kart at launch.
buy zelda, play it, get bored with it, save up new money, move on to mario kart