This is for a few reasons:
1) economy, people are upgrading necessities, or adding 'cheap' accessories, while keeping working products around longer.
2) Win7. Why buy a new PC when you can just upgrade Vista to Win7 and get more power from the computer? We are going to see the same problem with Win8. It runs on slow hardware better than older OS's (provided you have enough Ram... which is cheap).
3) Cheap upgrades. Ram, CPUs, Cases and Monitors are all relitively cheap as they have lowered their prices to meet the requirements of a slower economy. The only exception are HDDs, but many (myself included) are switching to small SSDs when our HDDs fail (so long as they meet our space requirements)
4) Tablets and Smartphones. While I still insist that they do not replace the use of a PC or Laptop, they are a reason to not get a new one. So long as your main computer is faster than the tablet or smartphone then why bother upgrading? With quad core processors, Win8, Office for ARM, and other changes coming this year I think we will see Tablets replacing PCs, but not yet.
5) perhaps most important, system requirements. If all you are doing is browsing the web and checking e-mail, and editing the occasional document then you dont need anything more than a late gen Pentium 4 or Pentium D, and a Core2Duo with a decent Ram and HDD/SSD loadout is about as fast as you can get for web browsing and similar activities. Even video games seem stuck; Froever tied to game consoles that refuse to change. It is only the content creation software that needs more power, and for the most part a mid-grade i5 system is more than enough, and the new i5's and i7s are largely overkill. So until utility costs rise through the roof prompting the use of newer more efficient computers (my new i7 rig with a GTX 570 takes less power in daily use than my old C2Duo rig paired with a 9800GT), or the natural attrition of dying hardware kicks in then we will likely see this trend of lowered computer sales continue. But I still think most households will have at least 1 PC for the next 5-10 years, which will later convert into a home server for a multitude of mobile (smartphones, MIDs, laptops, etc) and fixed use devices (refridgerators, TVs, HTPCs etc.)