Here are my speculations as to why people don't reply to your online content (blog posts, Twitter tweets, G+ articles, etc.), based on my observations and my research on this topic. I think this is the most complete listing you'll find on the internet.
(1) Inarticulate Not being good at expressing themselves, they don't even try to respond, though they have strong opinions.
(2) Fear They're afraid of saying something that others may consider stupid or extreme or silly.
(3) Busy People are generally in a hurry when they're online. They don't have time to read a post, think deeply about it, mentally formulate a valid response, then post that comment at every article they read.
(4) Lazy Some people are under-achievers. They do just enough to get by. If it's not mandatory, they prefer to skip it. Posting a comment is work. They don't like work. They like leisure.
(5) Dumbfounded You have stated something so completely, so intelligently, so funny, or so profoundly, they are astonished, at a loss for words, and cannot think of an adequate reply that would add anything to what you said. They think that anything they have to say would only detract from your brilliant article and look bad in comparison to what you wrote.
(6) Lurking Some people have no intention of interacting with anybody online. For a variety of reasons, they prefer to just read and move on, collecting data, absorbing information, which they may then disseminate in the offline world.
(7) Passive There are sluggish people in this world who have been trained by TV, spectator sports, and Hollywood to passively soak up entertainment. They don't participate, they are sponges.
(8) Drunk Some folks are wasted when they read your post, and you should be glad they refrain from posting a comment on your articles.
(9) Shy There are people who don't like the spotlight. If they posted a comment, they're afraid that would attract unwanted attention to themselves.
(10) Non-argumentative Some people feel that by posting a comment, it would be an invitation for someone else to start a debate with them, and they hate arguing, or are not very good at heated discussions. It's similar to those who fear public speaking.
(11) Not smart I want to say this nicely: some people are not intelligent enough to understand your content, and don't want the whole world to see that.
(12) Irrelevant To some people, your content is not relevant to them, so they may read it, or skim over it, but are not very interested in the topic, and thus, refrain from adding to the conversation.
(13) Wrong field If someone is not educated, trained, or experienced in the specific field related to your article, there is no point in them trying to saying something, or pretending to know something, on this topic.
(14) Bad presentation You may have written your article in a manner that offends the person, so they punish you by not posting any comment. You were rude, extreme, angry, vulgar, haughty, arrogant, prideful, condescending, silly, dopey, or too intellectual in your style.
(15) Controversial Your article is too inflammatory, negative, political, religious, sexual, edgy, or radical for them. They may admire you for expressing your opinion, and may secretly agree with you, but don't want to interact with the article, for a variety of reasons. For example, fear that their employer will read their comment and not like it.
(16) Non-interactive You never or rarely respond to comments, and they see that as being a reason to not interact with your articles. Can you blame them?
(17) Comment begging You plead with people to post comments, and your neediness turns them off. You sound too desperate. They interpret it as fishing for compliments or an insecure seeking for approval.
(18) 3rd party comment apps Count me in as one of those people who can't stand Disqus, LiveFyre, Haloscan, and other third party comment management applications. They don't want to take the time to sign up and create a password for yet another account. Using such services will severely limit the quantity of comments you receive.
(19) Login Many people don't want to login to post a comment. If you forcet them to login with their Facebook, Twitter, Google, or other account, you are placing an obstacle in their path. Just require a valid email address to post comments.
(20) CAPTCHA There's really no valid reason to force people to interpret a bunch of squiggly letters or numbers, then enter them correctly in a text entry box. Just moderate your comments to filter out spam and abuse comments.
(21) Anonymous trolls welcome If you allow anonymous comments, with no link to the comment poster's blog, this will turn people off, and also could cause them to fear retaliation, since most nickname and anonymous comment posters tend to be trolls, abusers, and jerks.
(22) Impersonal Your post has no personality. It's too scholarly, inhuman, textbooky. The cold, aloof style leaves them with no motivation to interact with you.
(23) Elitist You have an insider group that interacts with you via comments, but they feel like an outsider. They don't want to be seen as an intruder. They're afraid of interrupting what seems to be a party to which they were not specifically invited.
(24) Unasked Some people will post comments if you ask for reactions. Not beg for comments, but simply say "what do you think?" or "what have you experienced in this regard?" or "what do you recommend I do in this situation?"
(25) Infrequent posts You don't publish posts very often, so people think you aren't really into engaging in conversations, you just post sporadic articles to build a body of self-originated content.