Wii music is part of the Nintendo line of wii tech demos. It is meant to demonstrate the uses of the wii and nothing more. In fact, the enitre function of the wii series of games is to find new uses for the wiimote tech, and demonstrate them with the best motion and infrared controls possible.
Like it or not, the wii has the combined sales of the PS3 and the Xbox 360 entirely because of its superior control scheme. Not just the waggle, but rather the remote control design, the self explanatory interface (point and press the big button in the middle), the infrared technology, and the unique and comfterble way the nunchuck conjoins to the wiimote. However, the selling point of the wii has always been motion controls, and to get these, one must buy a wii game, such as wii sports, wii play, wii fit, or wii music. With so many games that refuse to use the wiimote the way motion controls should be done, and a huge install base created entirely by motion control implimentation, it should be a surprise to no one that games such as wii-music sells 2.5 millions copies; most of the other efforts fall flat on their face in this reguard.
There will probably be more core games when intuitive motion controls meet a core game, and that game sells better than a tech demo. Mario Kart wii is actually a good example; love it or hate it, the controls are there, and it keeps up with wii fit.