I tend to think that part of the problem is that there is simply way too much taking up our time today. People are interested in communities like this but they are also interested in other things, including making a living. It's easy enough to come up with a quick reply to a post like this. It is another matter to start with a fresh idea on a specific theme (like blog sociology) and compose an original post.
And it doesn't end with just the post. For a community to really be active people also need to respond to comments and turn them into discussions. Over all, I think the whole process takes more time than most people have available for something that is of marginal interest.