A while back when I was on staff as Tech Admin we made a decision to set the flag for a timeout on editing after a certain period of time (I think it was 24 hours from original post, can't remember for sure). Unless Tim has since changed it, it is still set that way.
The reasoning at the time for it, we had some members that posted stuff, and then for some reason would go back at a later date and clear out their posts. While this happened in all sorts of topics, it happened most where someone posted something scathing about someone else, and thought better about it a couple days later. In the whole, it presented all sorts of broken threads if you will. Major disruptions in thread conversation continuity.
We elected at the time to put the edit time limit on for two reasons. 1) To ensure post continuity in the future (even the distant future) for discussions and 2) To promote folks to think on it before laying out a scathing post, knowing they can't edit it in a couple days.
We had tried not allowing editing at all, but after quite a few negative comments about that, and realizing there was a fair need to be able to edit your post for spelling and grammar and such in the short term, it was put in on a time limit.
Anyway...that's why it is (or was) there.
