Honestly - because Discourse is pretty awesome and the folks behind it are very cool. By paying for hosting with http://www.discourse.org/ we also help fund development of a bit of great open source software so that's also a classy thing to do regardless of the language the developers happen to choose to develop it in.
There are forums modules for Orchard (which is the CMS that we run the .NET Foundation website on) and there are stand-alone forums applications written in ASP.NET. There is also the forums module from CodePlex that we could have approached Microsoft to see if we could have pulled out, open sourced and used. But we wanted something hosted and kept up to date by someone else and Discourse as a service fit the bill nicely so we didn't look much further than that.
If you have a solution that you think we should consider let me know. So far I've been pretty happy with Discourse - love to know what everyone else thinks.