Obviously the stuff that's already been mentioned (Wii on the decline & towards the end of its lifespan, SS needing a hardware add-on etc.) contributed to it, but just from colloquial evidence: I think people are getting a little tired of Zelda.
Myself, and a few of my old Nintendo-loving friends bought Twilight Princess at the time (and enjoyed it quite a bit), but I just don't really have any desire to play SS, same for them I assume. One of my friends who did play Skyward Sword told me that he "Enjoyed it but it wasn't really anything new".
I read a lot of stuff a while back about how Miyamoto wants Retro Studios to work on the new Zelda game. I think that's a really wise move. RS has, from what I've seen of their games, a knack of keeping things faithful to the series but making them feel fresh and new - and that's what Zelda needs most.
If I'm wrong and Skyward Sword is the Zelda revolution that all the reviews said it was, then just ignore this I guess :)







