The people playing this on the Xbox or PS3 generally don't have $4,000-$6,000 lying around the house. I'd bet people that play on the consoles are the middle-class people who can't afford the PCs, otherwise I'd be playing it on a PC.
I'm actually what is considered an upper class person, but that would be if mooching off of my mother's financial standing counts. My mom has always known the value of money, having grown up as a war zone as a lower class citizen, so she never wasted much money on what she considers impractical things, including a gaming PC (and I sure don't know how to build one, nor would my mom let me try) and after being used to playing Playstations for so many years, I don't really want to waste money on one, either. I prefer my Playstations. I don't need to mess with them to get them to work, it's connected to my TV at all times, I've got free Netflix movies available for instant viewing, and it's a Blu-Ray player. Even after completing my current plans of attending medical school after college to work as an internist, I plan on using Playstations as my main platforms of choice.
I also know of many middle-class people who have gaming PCs, although those people (the ones I know) typically build their own computers for much cheaper.