I have always had the (most likely biased) idea that the Microsoft platform is, in general, lower quality when compared with other options (mainly, Unix-based) - specially on the server-side.
Why does Stack Overflow use Microsoft technology?
Perhaps the easy answer is: because, just like you have a personal preference for Unixes (most likely due historical factors), so have the creators of such site.
If that is the case, then my question would become: why or how did the awesome authors/creators of Stack Overflow become Microsoft lovers? Is it just because they wanted to have more job opportunities/coverage or because they truly admire MS products?
I would be very interested in reading the story.