Not the physics, but the markets
I think it has more to do with supply and demand than technological prowess. The developed world is a mature market saturated with PCs: Few first-time purchases, mostly upgrades, and there's a lot less pressure to upgrade the PC you already have than to buy a PC when you don't have one. The virgin markets are in the developing world, where, going from zero to anything, there's little market for bleeding-edge and expensive PCs. Clock speeds from two years ago are still infinitely better than none.