Just look at the NES generation of games to know why Wii Sports will still be popular in 20 years and Mario Galaxy won't.
It is not shallow in terms of gameplay and the content presented in Wii Sports Resort is quite huge. And it makes the game better because the definition of quality for the people who play Wii Sports is that a game should be easy to pick up and play with your friends. That means to a person who wants to play a game like Wii Sports the quality of a game decreases the more complicated it is to pick up and play with your friends. Basically put Metal Gear Solid, Final Fantasy, Mario Galaxy and Zelda Twilight Princes are... utter crap to a lot of people. And they are in the majority 
Why do you think you know what quality is but a person who likes Wii Sports doesn't?
Do you know how quality is defined in business? No offense here, I'm just asking. Do you know what the definition of quality in business is?
