The brilliant part of Apple's strategy is that they control the hardware, so they can afford to do a maintenance/64-bit prep release like Snow Leopard that's light on new features because there's no bargain basement Mac clone shop to eat into their sales if the new OS release requires less hardware.

With Microsoft there's a perverse incentive to keep upping the system requirements. I'm not saying they do it consciously, but if this theory of why no one upgraded is correct then it puts MS in a very difficult position.