There is a stickied comment explaining the rules and culture of AskHistorians in that thread.

You should check out this stickied explanation for why so much is removed int the thread you linked.

I disagree. Only by enforcing such a strict standard on the responses can a subreddit like AskHistorians ensure that those who are prepared to answer questions do so with sufficient reasoning and background behind their arguments. There are plenty of other history subreddits where this is not required, so I would argue that actually is what is good about this subreddit. And the mod has posted an explanation in the question :)