The real reason why they have a hard time selling is not because of the installed base being "casual", or that Nintendo only is successful on Nintendo systems. the weight lies more on the developer/publisher perspective.
- A developer can make a very good core game, but the publisher doesn't market it properly or does a pretty poor job.
- A developer does a very mediocre-poor excuse of a game.
Another cause (I'm analyzing it for my blog) lies in the retailers that do not help a thing