Because it's in the guidelines:

"Off-Topic: Most stories about politics, or crime, or sports, unless they're evidence of some interesting new phenomenon. Videos of pratfalls or disasters, or cute animal pictures. If they'd cover it on TV news, it's probably off-topic."

See also: https://news.ycombinator.com/item?id=6120530

Key takeaway: we can comment on politics as much as we want (I recently signed up for strongtowns.org !) but here is not a good place for it.

I disagree. "Most stories about politics [...]" implies that some are ok. The criterion is "[...] unless they're evidence of some interesting phenomenon.". As the Brexit is indeed a precedent, I would argue this story isn't off-topic. The amount of comments underlines this conclusion.