Soundwave on 24 September 2013
NSMBU is the main pack in game, think of Wii Party U + Wiimote as just an extra throw in.
Wii Party U/Wii Fit U probably didn't have very large development costs, which is why Nintendo is more willing to give those away in a bundle.
I could be wrong but I think this is the first time Nintendo has bundled a Mario game with a Nintendo console in Japan. Usually Nintendo never does that in Japan, but desperate times call for desperate measures.
