Here are the three main reasons why it's going to happen. If you are too lazy to read, just skip down to the summary.
1. Casual games sell a lot, while core games don't
Everybody knows how much casual games like Wii Sports, Wii Play, Wii Fit and Mario Party 8 have sold, while core franchises like Metroid Prime, Batallion Wars II and Wario Land sold much less in comparison, some would even consider them to have bombed. Other examples of failed games include this, this, this and this.
2. Nintendo has a short term strategy
Since they stroke gold with the new casual market on the DS, Nintendo is milking the market of uneducated customers for all its worth with the Wii. This bubble may eventually burst, but Nintendo couldn't care less about this, because they could always simply retreat from the gaming market and fall back on their other lucrative divisions like Pokemon plush dolls.
3. Core games have no benefits
These games are expensive to develop and take years to make. The only reason Nintendo ever made these games were to get great ratings from critics. However, the new casual market doesn't even read these media outlets, so core games have become a complete waste of money for Nintendo.
Bottom line: If Wii Music becomes a success, there won't be any games made for us anymore. Nintendo's future depends entirely on this game. Everyone should feel offended by this game's existence.