I wonder about this too. Sometimes you come across thought provoking articles that cry out for comments, but there are none. If the article is already a few hours old and fallen off the front page, and you do post a comment, it probably won't get a response. This greatly decreases the incentive to comment (since what you want in the first place is to discuss it with others).
I now try and leave a comment if I find a good article that has slipped through the cracks.