Well there's a few reasons why this game wasn't liked. One of course was the game wasn't an amazing game by any standards. Sure it was an innovative way of looking at the genre and definitely something to build upon but it needed a little more substance.
However, that doesn't logically make it a bad game which gets us to the next point. There was simply a dislike for drifting away from the GH/RB style. Even though to get those games you had to have a radical change, no one wanted to change the setup for something "potentially" better. Of course this works as a really huge slap in the face contradiction as many people talk about the over saturation of GH/RB and "tired" of the formula. Thus most of us who have looked at the case don't really consider this response to be a credible one and thus could be one of the reasons why they were "shocked".
Finally there was just a general outcry against Nintendo's 2008 holiday lineup. Give or take it wasn't the hottest lineup by any standards with Wii Music and Animal Crossing, there was general dislike towards the lineup. Wasn't warranted by anything they could base off Wii Music but simply because this wasn't what they wanted for their October Nintendo title.
Could also make an argument as a backlash to a seemingly new focus on casual but I think that kinda ties in to reason 2 and 3. Not to mention this was never considered a credible reason and generally overlooked by anyone in the first place.
So what ya get is that Wii Music was probably a decent/average game. Nothing spectacular (like we usually get out of Miyamoto) but in no way one of the worst games of this generation or really deserving of a lot of the hatred it got. So I guess they should be a little shocked but it's easy to understand. But they shouldn't be shocked it didn't get high reviews either.
I would really like to see a sequel to work off the idea and give us something that fills in the holes of the first. It was a great concept and truly innovative but needs that substance and depth that it was lacking. I'd love to see Miyamoto and the team go back and do that.
