One possible reason is that computers are being built so much more efficiently because of manufacturing advances.
The truth is that computers are being manufactured at a higher ratio in relation to the demand for them. People often buy new, high-end machines when their old one becomes slow or starts having problems, but there isn't enough time in between upgrades to allow the computer stores to restock. Secondly, studies have shown that people who learned how to maximize their device's efficiency (through programs like firefox) tend not to replace it as quickly and only really get concerned when it's actually damaged or lost--even then they might get a replacement on Amazon. Thirdly, IT departments need robust backup power sources...