A lot of indy devs are using Unity. Hell, a lot of non-indy devs use it. So that's not a big deal.
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.
As the OpenDiablo2 developer has confirmed: cross-platform C# is a pain.
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.
