Speaking as a casual gamer, I would say that the problem is the same as making children's films or television shows. All of the really good ones appeal to both children and adults. When they try to specifically target a younger audience, they lose the adult audience and thus make an inferior product, a product that only has a very specific audience which it may not get. see: "Quest For Camelot
So it is with games. The best casual games appeal to the more hardcore players as well, at least as an in-between "real" game diversion. When developers try to specifically target the casual gaming audience, they tend to just dumb it down to the point that it appeals to nobody. Because, get this y'all, casual gamers are not stupid. They just don't devote long hours to playing games. So games with high learning curves, difficult controls, obtuse concepts, etc do not appeal. Neither does games that require a high-time commitment. We don't have that kind of time. That's why we're casual gamers.
But this does not mean we dislike a game with depth and complexity. In fact, if a game manages to appeal enough for us to sink extra time into it but it turns out to lack real depth, we feel just as cheated as the rabid hardcore gamer. It might take as a little longer to get there, but we probably feel even more cheated since our time is precious. A game without depth feels like a waste of time.
My point is, there are so many factors that goes into making a game and how a game appeals to an audience, that it's pure hubris on the developer's part to try to appeal to a specific audience, as if they really understand what will appeal to that audience. Spore, for instance, does not interest me. Neither does Sims. That either of these games has any popularity just proves that I'm not everybody. I'm just saying that regardless of the gameplay structure, neither one of those titles are what I wish to do with my time. So right there they don't get the entirety of the casual gaming market because I will not give them any money for those games. So, they have a smaller subset of the casual gaming market, because I can't be the only one who thinks Spore can go fuck itself. That Sims also appeals to the hardcore demographic is why it managed to be successful, and if Spore does not if the OP can be trusted, then it will most likely fail.