I would imagine that it's due to the way in which the Xbox was developed.
Just like the Zune, Microsoft rushed the Xbox to market. It used off the shelf components and thus they managed to develop it in a very short period of time.
The problem lies in the fact that Microsoft doesn't own the hardware. The original Xbox was essentially a PC and all the component manufacturers charge Microsoft hefty licensing fees.
That's not normally how consoles are produced. Sony own the rights to the vast majority of the components inside the PS2. There's no third parties demanding licensing fees and they can do with the technology as they please (i.e. develop it further or miniturise it). Any manufacturing cost savings can be passed on to the consumer.
Microsoft have gone down the same route as Sony/Nintendo for the Xbox 360, so I wouldn't be surprised if we see a Xbox 180 towards the end of the 360's shelf life.