I think most people simply aren't interested in what this game has to offer. It's likely most people don't even care about making music, but rather pretend like they're capable of it (Guitar Hero does this extremely well).
Wii Music doesn't even have any set goals, which doesn't help. This is why people make the "it isn't a game" argument, because it holds some truth. It has more in common with a musical instrument than videogames, so it's no surprise most core gamers aren't interested. I think the difficulty is irrelevant either way, there are surely some people that can play the game well, but still be uninterested.







