Unfortunately there are a lot of people who want something for nothing and they don't like to (can't be arsed to) share their material or participate. I've noticed a HUGE increase in people who aren't into music production, but they want stuff by the latest pop princess so they can lord it over others in their peer groups.
Also, there are members on here who will just download files and then share them on soundcloud/reddit/tumblr/youtube etc instead of sending people over to this site.

Because there are more "collectors" and fewer musicians/producers/remixers, there's less work being showcased. I think fear of copyright problems may also be a smaller contributing factor.