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.

Mono was in a usable state on Linux for literal decades before becoming official and integrated into what is core today, that is unless you needed windows forms, which much like MSFT UI frameworks today had multiple failed attempts spanning those same decades...