Was WW generally expected to bomb? I know it's a remake but I'd imagine many kids haven't played it before, making it new to them.
The original sold something like 3.6m initially, eventually skirting along to about 4.5m -- which is better than Skyward Sword but worse than Twilight Princess. That *was* on a userbase a 1/5 the size of the Wii, so it can probably be considered a good performer, however there are currently more copies of Gamecube WW than there are Wii Us in the wild, and no matter what improvements you make - its a game that people have played before. If even a fifth of the current userbase bought it in the coming few weeks (as on Gamecube) - that would only be around 700k worldwide.
I suspect it could be falling quite short of that... for a 'big' game that encourages Miiverse participation (tingle bottles, pictobox sharing), and has been out a week, it only has 39300+ participants on the European side. Obviously that's not a great metric, but for a Zelda game I expected a little better.
It charted better than I expected it to in the UK, even though it came third behind heavily falling FIFA and GTA releases, but NPD will be a better bellwether