Historical note that Arena Wars, released in the .NET 1.0 days, with thin bindings to OpenGL, was the very first .NET game, years before XNA.
replyhttps://en.wikipedia.org/wiki/Arena_Wars
C# being closer to C++, and .NET having direct support for C++ helped quite a bit regarding adoption among game studios.
Unity started on Mac, and they only adopted .NET when doing the cross platform rewrite.
I also don't remember if there wasn't some Mono advocacy at the time, as they became customers in the process.
Ironically Mono/Xamarin is almost gone from official .NET, all these years after the acquisition, with every modern .NET release, another bit falls off.
We are already on the phase that CoreCLR might take the remaining bits.