The problem is that if you leave the deletion and edit windows open forever, people will come back and abuse them in order to change the meaning of threads after the fact. Sometimes, and unfortunately not even that rarely, they do it maliciously, for example to make repliers look bad. So there needs to be a balance between allowing edits for a while—for legit uses like fixing errors, clarifying, adding new information—and not allowing them forever. We can argue about what the length of the window should be, but a few hours seems like a fair balance to me.
It's important for the coherence of HN threads that discussions proceed in a mostly-append-only way, and that the history be sort of 'committed' or 'trusted' after a while. That doesn't mean that nothing can change, just that at a certain point, changes need to go through a manual process, as I've explained in other comments in this thread.