I think there are many reasons. Joel Spolsky and Jeff Atwood were both hugely into .NET/C# and Windows Tech (one of them worked at Microsoft even maybe?). Then the entire startup was built with Microsoft tech, because they knew that best, so when they talk about their architecture they obviously draw a particular crowd among others. It felt like a lot of the early community came from the founders blogs and Microsoft circles, and I was also asking mostly C# questions on SO too in the beginning.

Probably someone remembers the history better than me and have more clarity and/or corrections.