This is probably because the articles which are interesting people a) upvote and b) don't understand well enough to comment on. For example, if we were given a

This is probably because the articles which are interesting people a) upvote and b) don't understand well enough to comment on. For example, if we were given an article on protein folding, it might be very interesting - causing upvoting - but also difficult to comprehend for many of us - causing a lack of comments. A nice heuristic. I think I'll try it a few times.