Because the first one sold on word of mouth not advertising.
Also sequels typically do lousy on Wii. Most people are content with one version of the game.
Personally even though I didn't care for CG I'll rent NCG at least to try out the WM in it, but I don't think that'll really move any copies. I don't think Wii's casual base actually care if it's 1:1 or not (don't tell Sony...).
Also, when CG launched it was the FIRST mini-game collection aside from Wii Sports/Wii Play and it was decent. Since then there's been... lord knows... hundreds?... of craptacular mini-games, some even cloning CG. The market for this type of game on Wii is WAY over-saturated.
There's hope for it though. IF it's really good, word of mouth will once again spread and sell millions of copies.
