I'd reckon the majority of users, you see this all over the internet where there's ability to vote or 'like' something; Twitter, Youtube, Facebook etc - the vote will usually outweigh the comment count.

On HN the culture of 'nothing good to say?; say nothing' is fairly baked in so will create this effect even more so.

Voting allows lurkers a voice, who often are the majority of users (https://en.wikipedia.org/wiki/Lurker citation 11).