Working on C# OSS means stealing licenses from your employer, buying an MSDN subscription on your own dime, or evangelizing enough to get an MVP award so you get all the tooling for free. The free Express editions of the tools have built in limitations that don't make them viable for building Big Data projects. Other options exist, but pale in comparison to Visual Studio so they don't get much use.
As for mono, the uptick for the OSS stuff just isn't there and even Xamarin is leaning on commercial offerings to become a viable business. Maybe as they get more successful they can sponsor some C# server projects, but that may be a ways off.
This is all unfortunate, as C# really is a great language.