I think there's two factors here. First, the cost. If you're only going to offer 2-year-old hardware, it's hard to justify keeping the same price (especially when the base price was a bit high to begin with). Second, the Mac Pro is lacking hardware that would allow it to be future-proof (SATA III, USB3, Thunderbolt, new socket, etc). Basically, you're buying a dead-end of a computer at this point.