The ability to flag a story is given after reaching a certain karma level (I don't know the exact amount).

So the short answer: because the necessary number of readers clicked the 'flag' link. As to why they choose to do so, your guess is as good as any guess.

I got my first ever comment flagged a day or two ago. I asked if the goalposts had moved about the economy.

Politics are not to be debated here, they're to be dictated.

Say rather that he's apolitical

"Once the rockets are up, who cares where they come down?

That's not my department!" says Wernher von Braun

There are other people who see value in a forum like HN having a focus, knowing that readers who want to experience general outrage and spectacle already have plenty of options.

People in the latter group tend to flag stories posted by people in the former group when those stories plausibly violate community guidelines about politics and the type of general interest slop that is available everywhere.