Well for me, that's like asking why don't you write everything in Erlang rather than C#.

Go is for me just a, hmm, concurrency programming language. Its popularity stems just from being pushed by Google, and the cosmetics of using braces. Despite what pundits say, it's completely and utterly not like C.

While C# is a successful and comfortable everything-language. It is blazing fast, has amazing tooling, you can get going to prototyping, then enterprise code, its serverside handling is excellent. It has many low level features and built-in data structures. It's I would say preferable for almost any kind of work except scripting. (I really don't like C# scripting.)