Don't get me wrong, Nintendo are still very successful in my eyes. If I remember correctly, the top ten best selling games of all time are from Nintendo (citation needed).
I do agree that there are good games out there that people don't want to play for whatever reason. But there ARE reasons. Not only does Nintendo cater for a different audience, they're also an entirely different culture (Japanese, IIRC) and that style has never appealed to me. I find most JRPGs really boring, for example. Also, do realise that I am naive, and that everything I say on this subject is based purely on my young perception of the market, and not based off solid facts and statistics. And from what I can tell, AAA games are big budget, commercial games.
I just think Western gamers prefer different, more mature things. Again, correct me if I am totally wrong.