Threads that get a reply go always go to the first page. The threads that aren't being replied go to the other pages and if no one search for them and do not reply they will have more chances of being deleted specially the ones in the last page. AHS has a limited space to stocking threads so when there's need to free some space the staff gets the threads on the last pages and delete them. Well this is what I think how it happens. The super old threads with no replies are probably deleted every week or two. (I may be completely wrong
so don't take it as a 100% truth since it's only a guess LOL)