C# has had the reputation of not being viable for Linux for a long time. Therefore, the people already on Linux didn't have a reason to use it or even try it. If you're already doing stuff in other languages it's hardly worth it to switch to C#.

I personally use it quite a lot - but I came as a windows user writing all my utilities in C#. Also, afaik C# is mostly used in corporate environments that don't open-source their projects. You're unlikely to hear from it unless you're working on it for this very reason.