Does anyone know why would you try to develop an opensource game in C# in the first place?
There are plenty of good reasons not to use C#, but we shouldn't pretend that there aren't plenty of good reasons _to_ use it for specific problem spaces.
There are some other frameworks that use C# and are cross-platform. Monogame is the cross-platform replacement for MS's XNA, which targets most of the major game systems out there.
Yes, I understand that Windows is big and there are a lot of C# developers/fans here who immediately scream "What? C# can do anything!!!", but in practice MS doesn't support opensource where it matters and when it matters.
BTW, there are a lot of opensource games written in C#. More than there are written in Go actually.