In general, Mac OS X and Linux support for most open source software is significantly better. Both platforms are much more friendly to developing without proprietary software like Visual Studio (well, actually, you probably need Apple's developer tools to do any real compiling, but those are just a modified version of GCC).
Having Linux compatibility is good because most Rails servers run some type of Linux. As to why people don't use Linux as much for development... well, the Mac interface is nicer.