Please keep in mind this is only my opinion, but I believe the decline in people commenting on blog posts is due to the extra steps they have to take in order to leave one.
I try to be diligent, visiting all the people participating in shared events, but I'm so turned off by the "captcha" requirements and now the "prove you aren't a robot" numbers and letters that I'd rather just move along. My main reason is my eye sight.
I don't see nearly as well as I used to...even with glasses, and those extra steps aren't that easy to see. I end up trying at least two or three times before I get one right, and that's even more reason to not go to the extra trouble.
The other thing that annoys me is comment moderation. I think it's insulting.
What I'd like to know...has everyone had problems with unwanted comments or comments so bad you need to moderate them before they appear? I've been blogging since 2007 and only had disturbing comments once when I found myself in the middle of a job I did for an author I had no idea was a hot topic on the loops. I learned to research clients, and never had another problem.
I don't have a "captcha" on my blog nor do I moderate, so I was just curious why it seems more and more people do. It's time consuming enough to trek from blog to blog and when you have to waste time entering letters and numbers in order to comment, I think most people, including me, choose to mosey on by.