Don H pretty much explained one of the good reasons for not allowing deletions by the general membership. In addition, TFL was and is intended to be a repository of good firearm and RKBA information. If members were allowed to delete their posts at any future date after a reasonable edit window, the threads could be full of holes and non sequiturs. In fact, whole threads could disappear if the originator of the thread decided he wanted to delete his post.
Admittedly there are quite a few posts that need deleting anyway, we'll do that for you.