Most of that is either subjective or irrelevant. I've put more hours into NSMBWii and I got both games the week they came out. The main problem is MKWii has little to no fun singleplayer, while NSMBWii is fun alone and with other players. If you'd said Mario Kart DS, then we'd have something to discuss.
"Now, a fun game should always be easy to understand - you should be able to take one look at it and know what you have to do straight away. It should be so well constructed that you can tell at a glance what your goal is and, even if you don’t succeed, you’ll blame yourself rather than the game. Moreover, the people standing around watching the game have also got to be able to enjoy it." - Shiggy