I think it is because the "STEEMit will replace xyz social network!" posts get a lot of upvotes. And, it would seem, so do the posts on how STEEM works.
Could this be a short coming in how STEEM works? Older posts disappear because after 7 days they aren't worth upvoting, so someone else make a post similar to older ones, whether they knew about the post or not.
Yeah I just touched on that in a reply to someone else, we need to work on the discoverability of content. Just because its older doesn't mean that its lost all its value.