You shouldnt underestimate the fact that for a lot of people, 3D Mario is just too hard. They don't have the patience to master the controls, and unless you do, the game isnt fun. (But very fun when you got it, of course).
My wife is an example. I think she is a typical casual/bridge player. She loves games like Mario Kart (and regularly beats me), she can still fire up Super Mario World or Donkey Kong Country from time to time which she played when she was a kid, and otherwise enjoys strategy/simulation games like Sim City and The Sims. I asked her to try Super Mario Galaxy, but she lost interest after 5 minutes when she struggled with getting the controls, and complained the camera was confusing.
2D Mario games are much more "pick up and play". You got left and right, a run button and a jump button, and that's pretty much it. If you die, the levels are very short compared to 3d platformersso it is easy to retry and get back to where you were.
Even tho we have had real 3D platformers for about 13 years now (since Mario 64) they will never have the broad appeal that the original Super Mario Bros. had, or continues to have today.
You have to remember that Super Mario Bros was the "Wii Sports" of its time, a cultural phenomenon, something for "everyone". No 3D platformers will ever gain a standing even close to that, they are simply too complex, and are for more dedicated players.
8 million are excellent sales, and the game is still selling 50 000 a month, last I checked. But I am pretty sure that New Super Mario Bros Wii will easily pass 10 million, maybe even reach 15-20 million.









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