One reason I, and others like me have looked down on .NET developers, is that a large number of them aren't any good. I've meet brilliant .NET developer, some of the most talented developers I know are .NET developers, but the crappiest developers I know are .NET developers as well.
Some developers are completely ignorant in respect to computers, the Internet, algorithms and new technology in general, and they are almost exclusively working on Windows and by extension .NET. These are developers that can make code work, but take Visual Studio away from them and they cease to be developers.
It's by no means fair, because as I said, some of the best developers I know are .NET developers. It's just that I've never see anyone working on Linux/Unix be completely ignorant other languages, tools and platforms.