There is no denying, skyward sword is underperforming. I think its due to 2 things...
Despite the Wii 96m installbase alot of people have since packed the console away and are now playing another system. Those who are still playing it, see it as mainly a device more mario and party/family games.
My younger brothers who loved the earlier games simply don't care about skyward sword. mayb its the artstyle