Maybe it's a case of me misreading causality, but hasn't anyone noticed mostly that C# / .NET gurus occupy the Valhalla of SO? I also see the maximum questions in this field, and a classic long tail from there...
Does anyone have any idea why?
Knowing C# and Java is certainly an advantage on Stack Overflow. There are obviously more questions to answer. On the other hand, there are also more people answering those questions - so there's more competition to give the best answers (and give them quickly).
Overall I suspect it is easier to gain rep from popular topics - but it's not quite as straightforward as "there are x times as many questions, so it's easy to get x times as much rep."
Do C# / .NET gurus have a massive edge on SO?
Only if you think it's a competition.
I would venture to guess because the Microsoft .NET platform is rather popular and a very common tool used.
So naturally, you would see more questions/answers and people involved with this.
Computer says homework is more lucrative
https://meta.stackexchange.com/questions/3486/why-is-subjective-the-top-upvoted-tag-for-questions
C# and .Net questions are certainly more prevalent but they are also less lucrative than many other tags.
I would think answering questions from any popular language/technology would give you an edge proportional to its popularity. C# just happens to be a popular language. I really don't see how this hurts someone who does not participate in C# questions.