Hardware problems and upgraders.
Regarding people regarding this as a "hardware problem", they might as well. Windows 7 (let alone 8) require so much more resources than XP, that they can either a) spend money on more RAM and possibly other upgrades, on a system that is approaching retirement age anyway or b) not do that and realize it'll be replaced soon. (Partially, upgrade cycles are simply unncecesary, but realistically the systems will start to become less reliable as fans fail and so on.)
Regarding: "Where did all the upgraders go?"
I've seen several big causes of them evaporating:
1) The same people that balk at Ubuntu or whatever being too different, see Windows 7 and especially Windows 8 and also find them too different. They are rather irrational and find ***ANY*** change compared to bone stock XP to be some huge deal.
2) They find out the REAL choice is buy a new computer, put Ubuntu (or something) on the existing one, or keep it as is -- these are not business users worried about a hardware upgrade lifecycle. But, tthey have a laptop or whatever where it will not meet 7's requirements, and is not upgradeable to meet the requirements either. I've upgraded 3 of these cases to Ubuntu (with Unity turned the hell off, replaced with gnome classic), after their XP install got totally pwned (and no reinstallation CD or partition of course). They could not be happier.
3) Even worse, people that were interested in getting a new system with 7, then see the systems now have Windows 8... at which point, they plan to NEVER upgrade. Yes, people would rather use a 10+ year old PC than Windows 8.