The simple reason is that enough HN members flagged it to trigger the "[flagged]" tag. The deeper reason as to why HN members flagged it is tougher to answer as

The simple reason is that enough HN members flagged it to trigger the "[flagged]" tag. The deeper reason as to why HN members flagged it is tougher to answer as it involves speculation. I think it's likely because (a) it's off-topic per the guidelines given that it's pretty much pure politics and using the guidelines' heuristic of "if they'd cover it on TV news, it's probably off-topic", this meets it; and (b) topics of this nature, regardless of their importance are very rarely constructively discussed on HN. That's not to say it's not worth discussing; some forums (virtual and otherwise) are better suited for productive discussion of a particular topic than others. HN members often flag such submissions even if they believe the topic is important for just this reason.