While Nintendo has been riding a wave of good publicity with their very popular Wii gaming console, they're not infallible. In fact, they've made that extremely clear with their release of Wii Music. While the game isn't terrible, it's definitely not the runaway hit that some thought it might be.
While music-based games have definitely been gaining in popularity, culminating in the Guitar Hero and Rock Band phenomena, Nintendo appears to have missed a key point. People don't primarily play these games to create their own music, but to pretend that they are great musicians. After all, most people who want to create their own music are likely to go pick up a guitar or some drums and actually make it.
While the game is still novel, and perhaps even enjoyable, it's definitely more one of those “hey, that's pretty neat” things, rather than a “I need to pay you $50 to try that myself” deal. Oh well, you can't win them all, and hopefully they can learn from this mistake and not hold on to it for dear life in denial.