I plan on buying this game soon. It looks like something fun for me and my girlfriend to play. It doesn't have to be a hardcore game, it looks like it's just a fun easy to understand game.

Why is that hard to grasp?

Advertised well and appealing to much of the Wii's audience

The best form of advertising is word of mouth, its what happened to the titanic and avatar, and this is what just dance has. Advertising helps generate word of mouth which generates sales. I think this game had a good combination of word of mouth and traditional advertising. Believe it or not i know a couple of people who bought a wii for this game.

The game is fun. The core just doesn't get it.

No amount of advertising can push the sales that Just Dance is having. It's an evergreen title, selling 100k+ every week along with Wii Sports Resort and Wii Fit Plus. Frankly, I haven't seen many advertisements on this game since January, yet it's still selling. Surely there is something fun about the game that is getting people to buy it.

Game journalists denounce almost all of the big sellers on Wii. For instance... Carnival Games, Deca Sports, and even Wii Play all had horrible metacritic scores but also had the biggest sales on the system. This doesn't mean that most people are stupid and like bad games. It means game journalists and core gamers have vastly different values from the expanded audience that primarily exists on the Wii.

Critics said that Just Dance was shallow and wasn't accurate and fair in tracking the player's score. The expanded audience took the shallowness as an easier game to into. The expanded audience's values are not so centered around "keeping a score" but instead are focused on having fun dancing with friends. For many people, this game was not about progressing through content, but about experiencing the fun of dancing.

The name really says it all... "Just Dance" and none of that other stuff that core gamers love.

Lady GaGa is hot.

Combination of advertising, word of mouth, and appealing execution. Take out one part of this equation and you have a game that sells much less.
Just advertising is not enough if the game doesn't appeal or is not executed well enough to create word of mouth (and before anyone says the game is not executed well due to, for example, not reading player movements well enough, maybe it isn't executed well by "core gamer" standards, but just go and read the Amazon user reviews).
Advertising IS critical though, a good game with no advertising will usually not do well either (unless it has adequate shelf space in lots of retails stores, something I believe for example MW:Reflex had).

she has a penis tho

Because most people find it really fun?

http://www.amazon.com/Just-Dance-Nintendo-Wii/product-reviews/B002MWSY3O/

dancing with stars enjoys top notch rankings worldwide.people want to dance or try to.Thats where the market is now.

... no.

No matter how much you would advertise Little King Story, Red Steel 2, No More Heroes 2, Call of Duty or any other core game it won't sell that good as Just Dance. Why? because Nintendo aims for expanded audience. And Just Dance is king of social gaming.

Just look at this... can you get a better commercial than this? I doubt. If I had a Wii this game would be instant buy for me.

http://www.youtube.com/watch?v=8mV0QS-flxg