There is an automated flame detection mechanism. Don't know how it works. probably some kind of count of downvoting of comments?
Users manually flag.
Most political stories are flame bait; the discussions are low quality.
Most politics is off topic, but if a story has been discussed and this one has for almost a year, posting more about the same story with little change won't add much to the conversation.
E.g. I commented about this story 10 (ten!) months ago: https://news.ycombinator.com/item?id=43212206 and do not feel the need to share my thoughts each and every time the story comes up again, although I did 5 months ago. Maybe in 5 months time I will do so again but not every day!
Users do not want to see the same story permanently on show for discussion - they want novelty.
Users tend to not like: Emotions. Propaganda. Accusations of conspiracy. Less thoughtful and more thought terminating comments.
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You can find better and more answers by the site moderator @dang on why things slip off the front page by using the search box at the bottom of the page.