As told here.

No real reason was given, but IGN speculates that SK has been "a very content-driven studio with strong beliefs in the elements of game creation including story, artistry, sound, and technology." (quoted from IGN) while " Nintendo has recently downplayed the role of technology in game creation and has been known to de-emphasize story, graphics and sound in favor of classic play mechanics."

So this means SK is free to work on any platform (and is capable, according to the article). I can imagine the possibilities :ats his Xbox::