It's not surprising, really, as Wii Fit targets a larger audience, while GTA targets a niche market.
As a side note, GTA hardly qualifies as hardcore. I know many GTA fans blindly defended GTAIV, only to be disappointed by its more realistic mechanics and casual, frequent save points being removed. That alone proves a large portion of GTA's fanbase only casually plays the games for some quick carnage. Any time I've seen someone playing a GTA game in person, they're always running around on free roam with a ton of cheats enabled, and not much else, many of which never even having started the story modes of the games.