A large number of comments are removed because a large number of comments are not up to the publicly posted standards of /r/AskHistorians. We ask people to maintain a certain baseline level of conduct in the subreddit. Many people choose to ignore that and post memes, jokes, racist remarks, links to Google's homepage, and similarly useless content.
Here is some discussion from the last time this was asked about.
The answers that are removed are really not something most people here want to see. They often get dozens of down votes within a few minutes of being posted. Trust me as someone who can still see all of the removed questions: You're really not missing anything.
Here is my stock respone for this:
The moderation team is very active and we are aggressive in our enforcement of the rules. Posts that break the rules of the sub are removed, and in cases of egregious or repeat violations, the poster is warned for their behavior, or even banned.
Some time ago, I did an impromptu study of removed comments in a thread that had gotten popular. I didn't save a link to the thread, but because I'm a very messy person, I still have the paper I did the tallies on, so I can give you a run down of what the deleted comments in a typical thread are!
Out of the 60 TOP LEVEL comments (this excludes follow up comments)
3 User Comments were visible
2 Mod Comments were visible
3 comments had been removed for being questionable or simply wrong.
1 comment was removed pending a request for a source
7 comments were removed for being one one or two sentence answers.
4 comments were removed for being personal anecdotes
2 comments were removed for being jokes
23 comments were posts that were only links, with no context. Mostly Wikipedia I would imagine, but I didn't separate what the links were in my notes.
13 comments were removed because they were comments asking why all the comments had been removed!
2 comments were removed because they simply made no sense whatsoever.
So as you can see, generally speaking you aren't missing anything. Hope that clarifies things for you a bit.
All the dead bodies can make the thread a mess to read. Why not just change the CSS to not display deleted comments?
I actually wish the mods took this a step further and deleted questions that are very obviously cut/paste from some kid's homework who couldn't be bothered to fire up Wikipedia or check a book out of the library.
Another pet peeve are the "I am XXX in XXth Century. What are my chances of Blankety Blank?" questions. A lot of the topic matter is actually interesting (there's a question on the front page now where the guy asks about US Army volunteers during WW2 in such a way), but those titles are so trite, stupid, and formulaic. This isn't a Choose Your Own Adventure title.
However, I think the overall quality of the sub is excellent, especially considering the 400,000+ subscribed users that make up the community. That's a lot of people, a lot of stuff to moderate. The mod team and community do a very good job of self-policing and keeping the bottom-tier content off the front page or just removed. Keep up the good work!
My question is why threads often show that they have comments yet when you open them there are nothing but comments removed? If they've been removed, shouldn't it just say there are no comments?