Why so many dead comments?

I see comments here all the time marked as “Dead” by mods that are insightful often GREAT comments.

Not flagged by users but dead. I don’t believe users are piling on as there is nothing wrong with them half the time.

So why? Censorship? Propaganda?

I will sometimes see a Dead comment then another one similar to it that is promoted.

It seems like HN censors strangers and promotes their friends or something