The former case makes a lot of sense if the issue is controversial or contentious, but I don't have a good hypothesis for the latter case. My guess is that people found the post useful or amusing but did not see (a) anything they wanted to argue with, or (b) anything they could expand upon. However, I don't have much confidence in this explanation because I am generalizing from one example (myself).

On a related note, I have noticed that making the first comment on a post generally nets you more upvotes than if you made the comment after many others have commented, probably because most people don't read every comment or get tired of upvoting by the time they get to the bottom.