Nobody is optimizing anything or pulling out the performance card. C# was chosen because that is what they were comfortable with when building the site.
Sorry, I don't think I was very clear. That's exactly what I meant. They didn't make a performance based decision when choosing their stack, and now that they're a huge site they're doing just fine - no need to go rewriting in the latest 'performant' stack.
It's not about optimization at all.