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?
Sometimes I won't add a comment because of the chilling effect of a future employer or online mob finding it and reading something into my words. It's not worth being contrarian on the internet, or there's no space for devils advocate anymore. Most of the time I will comment and it's fine though.
Sometimes, unrelated, I will write a comment and then when my thoughts are formulated, I delete it because I have benefited from the conversation and there wouldn't be any additional benefit or use from me posting it.