That is a good point, Skyward Sword is selling faster than Twilight Princess.
As to why the sequels of other companies do better than Nintendo released sequels; that's a good, I don't know. I am going to guess it has something to do with the amount of hype. Mario Galaxy, Donkey Kong Country, and Twilight Princess were three of the most hyped games in Nintendo's history, but that wasn't true about any of the sequels.
I describe myself as a little dose of toxic masculinity.







