Are you sure you're not trolling?
Well, since you said "always thought it was kiddie gimmick", as in speaking about the past, i believe your nephews going to 360 changed your view.
Anyway, there was this time, when there were "real" games, which were gameplay driven. It was before the time of linear "cinematic" (non)games and the flood of (EA:s) sports (non)games, crappy big budget movie/book license (non)games and endless series of sequels which didn't offer anything new compared to earlier parts of the series. There's nothing bad with that, except that those turned out to be the most of the market.
Now, you have had a taste on real games, which happen to have pretty wide appeal and which are definately build keeping in mind, that "everyone can play" and considering the variety of Wii games, Wii bringing something really new to gaming for "hardcore" crowd and lapsed gamers and it having an easy input, which is interesting for the non-gaming audience, you still wonder why does it appeal to people? I mean, 360 and PS3 offers only higher hardware specs and higher developement costs compared to last generation.