Anders Hejlsberg explains here: https://youtu.be/10qowKUW82U?t=1154. TL;DW:

- C# is bytecode-first, Go targets native code. While C# does have AOT capabilities nowadays this is not as mature as Go's and not all platforms support it. Go also has somewhat better control over data layout. They wanted to get as low-level as possible while still having garbage collection.

- This is meant to be something of a 1:1 port rather than a rewrite, and the old code uses plain functions and data structures without an OOP style. This suits Go well while a C# port would have required more restructuring.

C# has become a poor jack of all trades, trying to be Java, Go and F# at the same time and actually being a shity poor version of all of them. On top of that .NET has become a very enterprisey bloatware. In all honesty, I'm not surprised that they went with Go, as it has a clear identity, a clear use-case which it caters for extremely well and doesn't lose focus with trying to be too many other unrelated things at the same time.

Maybe it's time to stop eating everything that Microsoft sales folks/evangelists spoon feed you and wake up to the fact that only because people paid by Microsoft to roll the drum about Microsoft products telling you that .NET and C# is oh so good and the best in everything, maybe it's not actually that credible?

Look at the hard facts. Every single product which Microsoft has built that actually matters (e.g. all their Azure CNCF stuff, Dapr, now this) is using non Microsoft languages and technologies.

You won't see Blazor being used by Microsoft or the 73rd reinvention of ASP.NET Core MVC Minimal APIs Razor Pages Hocus Pocus WCF XAML Enterprise (TM) for anything mission critical.

It seems it's because AOT is a bit of a second fiddle in the dotnet ecosystem and native is a top priority for their case. After hearing the reasoning ( https://youtu.be/ZlGza4oIleY?si=1GKSX61AF20VQr-G&t=1000 ) I don't blame them for choosing Go.

C# needs an interpreter (.NET runtime) while Go compiles down to a binary. And the toolchain allows you to compile for other architectures fairly easily.

So that could be a fundamental reason why.