they didn't change it for the same reason that they didn't do nice design, IMO.

tagworld.com is really complex thing despite its simplicity from the outside. It has unique user base and handles around 1000 requests per minute. It uses around 20 web servers and 20 other servers for db, file storage, video/photo converting.

All code is written from scratch and .Net is great in maintaining the developement process. C# compiles and most errors get caught before deployment. In addition there is a buit in mechanism for validating dynamic pages, which also helps a lot. For some time we used to deploy the whole thing daily, with active users on the site.

.Net is more for the corporate world with lots of legacy code and outside apps. It just can't be done almost with any language except java.