I would agree with the Demographics argument.
The Wii may have sold over 50 million consoles so far but the buying intent of a decent part of those 50 million console owners is still pretty much unknown ( I'm speaking of those that purchased a Wii because of Wii Fit).
I don't think it has as much to do with the quality of the software offered on the Wii or a title like EA Active woudn't crush The Conduit or Punch Out in term of sales...
Lets face it, the overall game purchase pattern of the Wii owners isn't similar to the one the PS2 owners had last gen.......