Well it depends on the thread itself, I've allowed revivals of certain active threads if there's enough of an opportunity for discussion and so have other mods, while others may consider that the "discussion" aspect is over and it's best to start a new thread. But the main reason is that after a month a thread/discussion is usually considered dead and there's not much worth posting in it or posting in older threads with nothing to add instead of just making a new one and restarting the discussion with a newer/more interesting point of view. Apparently it was never fully explained why revivals weren't allowed, but rather an excepted rule (or at least according to someone a day or so ago) and it's something we've stuck with.
In Black and White, for example, the revival rule doesn't apply to Weekly Polls because we want activity there, but if they were made before the games were released, and based on speculation, then there'd be no point discussing them because you already know what happened, etc.
I answered that completely around the wrong way but I hope that makes sense.
