I can tell you the main reason most people don't switch. They have no compelling reason to do so. I know, because I'm one of them.
You can talk a lot about which is better, which is more reliable, which is easier to use, which costs less, which has fewer hardware problems.
I even took the plunge and bought a dual G5 a couple years ago. I tried, honestly. Some things worked better, but a lot of things were just different -- neither good nor bad, just uncomfortable. After the first couple of months, the G5 became a fileserver. Since I got a linux-based NAS box about 6 months ago, the G5 has been cold.
The "killer app" that would get me to switch is virtualization. But you'd first have to convince me that OSX offers a better virtualizaiton environment than VMWare ESX, plus game developers would have to support their products running in VMs.
Once that happens, then all you have to do is convince me to use a non-open semi-unix like OSX and not Linux.
8 Reasons Windows Users Don’t Switch