Why do you think it's surprising it's written in c#?

Previously low performance and a still low popularity?

Not sure what your definition or filters are for low popularity but c# and .net in general purpose and application development are very popular according to various developer surveys. Depending on the survey often in the top five to ten.

I wouldn't say it's poor performing, definitely not rust, c or c++ capable, but you can write performant code through various tools, processes and optimisations, or bind to libs written in other languages e.g. rust, c++ to have them handle with that must be incredibly performant. Unity is a good example .net C# and C++ working together to deliver a performant solution.

> I wouldn't say it's poor performing

Let me reiterate that I meant that it was previously low performing. I've developed in .Net since 2009 but only last few years have brought significant improvements in performance.

C# is quite popular in game dev circles because of the ubiquity of the Unity engine which uses it as its scripting language.

Sure, but presumably they did not write an emulator for the switch in the unity engine...