I believe Joel was a short time MS employee. And I think the MS stack was chosen because that's what Jeff Atwood and the other early folks knew best at the time.
Yeah, relatively short. Spolsky was a Program Manager on the Microsoft Excel team between 1991 and 1994 (he drove Microsoft's Visual Basic for Applications strategy.)
People seem to get a premium on their perceived 'time' spent at MS for being earlier employees, which often correlates with how important a system they worked on.