I am always wandering the reason of Digg's decline. At first, most of Digg's users were the ones from IT industry,or more detailed, the ones who were interested in internet, and articles digged were focus on internet and IT. So, it's very simple for one to find valueable articles who wants to read.
Then, more people knew Digg, and Digged many articles of all topics. So, there were too much spam for users who only wanted to read articles about internet or IT. And they went away.
It's also happened to the ones who were only interested in someother topics.
If users can set some tags for an article, and allowed viewers to follow a tag or a topic, he can focus on what he want to read. It would partly solve that problem.
So it's with QA. If there are too much posts with all topics(may be sub topics), it would be hard for a user to find what he can answer. Then, they would go away sooner or later.