I think there are a few reasons
1. Mac UI looks good, browser fonts look good there too
2. A lot of open source stack developers use OSX as their dev machines, so the code has already been tested there
3. Since OSX is BSD, a lot of the same server code runs, and runs well on OSX, so demos can run well with everything hosted on the same machine without needing VMs or other tricks.
Windows can do all of this too, but some stuff doesn't work as easily on Window. Microsoft server stack runs great on Windows, but 'the cool kids' are all running open source stacks.