You do realize that C# in VS is just a compiler? It's csc.exe. There's no reason you can't host it in Eclipse. In fact there are several 3rd party components that allow you to host a C# editor anywhere you like.

The problem, in this case at least, is that the C# experience is so solid that the motivation to create an Eclipse one is pretty low. Maybe the Mono folks have something for it. But the amount of work that MS has done for the C# experience is truly impressive. I can, and have, written C# programs in vim, but it's not something I'd like to do on a regular basis.