I think as you said it is a mixture of both.  The incentive just isn't there anymore to get an upvote on your comments or to upvote others comments.  I also think it has seen a decline because users have left.  I know when the HF was first implemented a lot of people made the decision to not upvote comments but instead upvote one of the commentors recent posts.  The problem with that is the majority of people who comment on my posts I already follow them and upvote all of their stuff :)