It's called business sense...If you are showing off a new idea, the best way to promote it is through simple and varied gameplay mechanics before applying it to more complex games...
Did Nintendo ever actually do this before this gen? With the N64 they went with full 3D and introduced the analog stick as a standard controller item and promoted the idea with Super Mario 64 which at the time was one of the most complex games ever made.
But from a business point of view WarioWare is a smart move because it's:
A. A popular seller.
B. Simplistic enough for non-gamers but the series also began prior to the term "non-game" even existing and thus has an established core gamer fanbase as well. Animal Crossing has similar appeal (or would if they made a NEW Animal Crossing game for a change) as it was accepted by the old gaming market but has enough similarities to non-games to be retroactively promoted as one.
C. Probably not a difficult or expensive game to make. The microgame design means one doesn't have to worry about level design or balance or enemy AI or anything like that. Come up with a bunch of one button games, group them according to similar themes and done. The hardest part is figuring out the quirks of the new hardware which Nintendo would have to do anyway. In this case WarioWare games with touchscreen controls and motion controls already exist which would save them that part of development. They could probably pound this game out in months.
In other words WarioWare is just non-gaming business strategy 101: make a product for less money and less time and sell it to a customer base that are not expecting better and make up a larger chunk of the audience anyway. It's the videogame equivalent of pop music and reality TV.