Earlier quoted context omitted.
Because for some teams it's easier to write tests in plain C# with NUnit/xUnit libs.
But why? Why shoehorn everything into a single technology? Is it so that certain people never have to learn anything else in their entire life anymore?
JavaScript as a server-side technology sucks. It was cool back in 2010 when NodeJS first came out and event loops were all the rage. But .NET is now 10x faster than NodeJS, statically typed, has better community support, better tooling, broader official libraries, better package manager, and as a bonus it's not JavaScript. The productivity and performance of .NET with Visual Studio is just unmatched.