Some people are, but you have to understand that the situation is very different from MacOSX...
With Vista, the *only* thing you get by switching to 64 bits is that you get the ability to use more than 3.5Gb as a whole and more than 2 Gb per application. And that's all, there is no performance gain at all, because there was no performance loss by staying 32 bits. If you don't have 4 Gb RAM or more, and if you don't use applications that can benefit from more than 2 Gb, you have no reason to switch to 64 bits. That's because the applications and the OS each exist in their own 2 Gb address space, so switching between the application (custom code) and the OS (API calls) has no extra cost.
As a result, there is little incentive to switch to Vista 64, very few people actually have 4 Gb of RAM. So, most people do not bother.
On MacOSX, a different architecture was chosen. The applications are not limited to 2 Gb in 32 bits, they can go all the way to 4 Gb. The tradeoff is that the address space is not shared. The OS gets the full 32 bits address space. And each application also gets it. So, whenever you have to switch between the OS and the application, and this happens quite often, you have to do a full context switch: save the stack, save the CPU state, switch, do whatever you had to do, restore the CPU state, restore the state. This is quite costly, even if the benefit is more potential RAM per application.
As a result, switching to 64 bits on MacOSX came with what seemed like a noticeable gain in performance - actually, this is seen from the wrong end, you're only recovering what you had lost due to the architecture choice.
So, for MacOSX users, there is a very real benefit from going 64 bits.
This explains the difference in the shift between the 32 and 64 bits version. On MacOSX, people made the move as fast as they could, because they could gain a lot in the process. On Windows, people only do it when they actually need it, because it's pointless otherwise. The Windows world will shift en masse when 4 Gb becomes the norm...