As others have mentioned the turnaround was due to offering a new kind of gaming experience. Had the Wii not had motion controls and instead simply been a next gen machine with hardware specs similar to the 360 and PS3 is likely would have been slaughtered and done Gamecube numbers (possibly even worse).
I think the breakdown for the new the Wii installed base will be something like:
20 million Nintendo hardcore
10 million Hardcore attracted to it because of JRPGs and other hardcore titles
70 million Casuals only attracted to it because of motion controls and the very casual games and products being produced around them (Wii Sports, Wii Play, Carnival Games, Wii Fit, etc).
You can really see it already in the games being sold for the system. For example early adopters tend to be the system's hardcore but with the Wii we already know that many early adopters of it are in fact casuals (check Wii Fit's sales in Japan compared to Brawl's). So if there is already a high percentage of casuals this early in the system's lifecycle the percentage of them later on as prices come down will only increase.







