Serious question: Why?
C# is faster, more productive, has better tooling, and generics.
Well, that's a highly-opinionated statement :P
The serious answer is that Go is a reasonably well-supported language at GitHub, and C# isn't at the moment.
Opinionated +1 :)
The C# compiler is called Roslyn which might not be obvious for people trying to find it on GitHub - it's incredibly well supported, regularly updated, and lively: