Is is just that you don't care enough to reply, feel like your opinions are unpopular, don't feel like you have anything to say, or something else?
For me personally, I just don't care and don't get any reward in the idea of conversing with some random person on the internet. The chances are, I'll never see you again on here or irl, remember your online name (I remember faces, not names), or you will have any real impact in my day to day life. This is how I feel about every commenter online. I don't talk to random people on the street, so why would I talk to random people on the internet? To me, there is little difference.
I value face to face communication much higher than text chat. I really enjoy reading peoples faces and expressions when chatting, to the point where I find talking to others online is the equivalent of having a conversation where everyone has a paper bag on their head when.
If I think someone is wrong on the internet, I just don't care. I read their comment, think to myself "They are wrong/an idiot" and get on with my day. I see no value in correcting a stranger on the internet.
I much rather have conversations irl, and I do, so I have no time/energy left for an online conversation. I'd much rather spend my time with my girlfriend, programming or anything else.
Everything feels so permanent online. I know I'm going to regret things I've said in this post in the future, my mind will change, maybe I won't lurk anymore after writing this!? The permanent nature of online communication goes the against how I should be as person, my opinions shifting and changing as I experience more of life, not held back by some random thing I said on a forum 5 years ago.
When I post, I open myself up to being attacked by people online. I can avoid this simply by never saying anything online.
To illustrate a point, I finally got round to creating an account on stackoverflow about half a year ago, and I still have only one point. I just wanted to vote on questions, but I can only do that if I have at least 15 points on my account, and that requires writing comments/questions (I think). For me, that means contriving 15 comments that succeed at playing some social game, that I simply don't have the time or energy for, so I've given up on ever voting on stackoverflow.