Maybe because 'hardcore' is a terrible word. They are not trying to 'dumb down the experience' at all. All that Nintendo is trying to do is make gaming accessible to a wider audience. You yourself said that Nintendo fans' favourite games tend to be the Marios, Zeldas, Metroids, Smash Bros.', etc.
All that Nintendo is doing is placing a lower barrier to entry in these games. Take Metroid, for instance. The concept of aiming a gun by pushing a stick in an arbitrary direction is understandably foreign and confusing to people who, until this point, haven't played many games. However, everyone understands pointing and shooting. That game received high scores from both core gamers and people who don't game much but are getting into it.
Zelda and Wii Sports Resort--swing the Wii remote like a sword, have the character swipe with their sword accordingly.
Mario Galaxy--spin the Wii-mote to have Mario do a spinning attack
Smash Bros--press a direction and a button to do an attack in that direction.
None of these games are considered 'casual' by anyone (except maybe Sports), and yet the casual gamer can easily pick them up in a couple of minutes and have a good time.
There's a difference between a 'casual' game (also a terrible term), like the fantastic Wii Sports and Rayman Raving Rabbids, and a badly made game like Wii Music. However, musicians that I've spoken to say that Wii Music is a much better music tool than say, Rock Band.
Really, it's some of the idiotic third party developers who think 'casual=stupid'.







