I suspect if asp.net MVC hadn't been out, it would have been written in webforms.
I seem to remember asp.net MVC had only been out a couple of months when they started writing it or not even fully released, I was learning django at the time because MVC so was so obviously better than webforms. But because c# was so much faster than either ruby or python, when asp.net MVC turned up, and it was very, very good for a v1, I stuck with c#, as did many other .net programmers I suspect.
At the time SO was written rails and django were still new and slowwwwww, php was king and still mainly written by page without frameworks, Java who knows, and the MVC revolution was just starting.
I'm not sure what you think he would have written it in? I have a feeling if you hunt through coding horror you'll probably find a post from the SO start time period saying "use what you know". In Jeff Atwood's case that's C#. I know that Joel's company had written their own language that I think at the time was still in use, so they wouldn't use the fog creek stack. Which was still based on Microsoft stuff anyway.
That said, SO was indeed a complete game changer in terms of the rapidity with which one could get solid information.
It might have been easy to go php > ruby, but C# > ruby meant massive amounts of pain or complety changing your environments. While my company at the time had VMs, they were all windows and it wasn't a case of simply spinning up an EC2 to play. The pain of leaving a MS stack was massive.
I'm using ruby/python a bit interchangeably as I was.playing with both, but even things like gem took a load of effot compared to now. Like a couple of days of fiddling doing nothing interesting, not programming but constantly hitting errors or tutorials not working as you'd got a step earlier in the chain slightly wrong. It was a huge barrier to entry.
The worst tutorials claimed to be for both windows and linux, but would link to an out of date windows installer and all the command line commands would be for linux. I once made the mistake of thinking I'd write little maintenance scripts in Python instead of VB, it was great until I tried to connect to a MSSQL environment, ended up in a world of pain.
Last time I span up node.js on a new windows computer it took me like 10 minutes. Everything has changed.
That said, I see what you're getting at now and it isn't what I thought you were talking about previously. It looks like what you're talking about is it taking a few days (or so) to get a working environment up and running vs. nearly instantaneous now? If so, fair enough I totally agree.
I thought you were talking more about the more nuts & bolts parts, like it taking a few weeks to come up to speed on Python (for instance) instead of several months. I've always felt that claims of it being difficult to change languages or other type things (e.g. changing from Oracle to MySQL) were radically overblown, and these days w/ sites like SO it's pretty much trivial.
These days the landscape is a lot better, especially when it comes to Ruby.
You're correct that ASP.NET MVC was new at the time, but with guys like Scott Hanselmann comparing it to an acoustic guitar production versus WebForms more full-studio approach, and with Phil Haack on the squad, and with the corporate blessing from Microsoft and promises of future integrations, this was not some fly-by-night project that required a lot of faith to invest in. It was a clear case where Microsoft got something right, thanks to a small team of talented devs.
Consider: for the first time in history, you had the ability to write clean C# code in a web context with full control of markup and proper separation of concerns, sans cruft or baggage imposed by attempting to stretch a "desktop" metaphor over an request/response medium. Don't underestimate the psychological effect of this on programmers who'd been spending their days in the trenches builting line-of-business CRUD apps with ASP.NET WebForms and unbridled VIEWSTATE. So to me, ASP.NET MVC was the clear and obvious choice for StackOverflow. If you accept that, what better language than the C# he already knew?