Because I have installed a hand-dandy “take that, flying spam-monkeys!” plug-in, which has automatically closed comments on older posts, but which allows certain posts that are older than the cut-off to stay open so long as they have ongoing discussions.
What I really like is that I can configure this plug-in to allow certain threads to stay open indefinitely if I really want, and I can still close comments off anytime I like on threads that get out of hand.
So, if you really wanted to post on an older thread and can’t, just let me know and I can open it for you! Or else post your comment here and we can discuss it separately to the older comments.
The plug-in is called Comment Timeout, and I have heard good reports. Let me know if you have any problems.