Apparently Microsoft knows that not too many people are keen on upgrading their Xbox 360 . A rough economy means fewer people have tons of disposable income lying around. Plus, the Xbox 360 is so much more than a mere game console: it streams Netflix, it downloads movies quick-like and, um, other stuff. (I still mainly use it as a video game console, for the record.)
So, Microsoft will instead focus on adding features and other things to the 360 rather than coming out with a whole new console. It saves you the trouble of having to buy a new console in a few months, and it gives Microsoft and its “content partners” some time to figure out how to maximize the console’s potential.