It could be that the Mac-based Rails developers are more visible on the web than windows-based ones, but I do believe that the Mac has an unusually large presence in the Rails world. (And I like that :) )
Why? I'd say that Textmate is certainly a factor, but likely not the biggest.
I use both windows and mac everyday. In windows there's always something bugging me... something crashed, or there's an update to the anti-virus, or some other notice popping up in the tray. The OS is always needing my attention, needing me to do something.
On the mac that doesn't happen. I just do what I want to do, without having to do stuff for the OS. I use the computer to work, rather than having to work for the OS. The result is that my productivity is consistently higher on the mac.
Take that, give it all the power of a UNIX/Linux system, and interoperability with UNIX/Linux, and you get a pretty darn great platform for programming.
