Well, I'm no spokesman for the moderating team, but the key thing seems to me to be the level of effort needed to keep things on track. Pretty much all of us agree about Trump and right wing politics - in that we at RAWK tend to despise what they do, Republican or Tory. Little dissent arises, the insults are aimed at the representatives of the right.
Left wing politics has, I think it's fair to say, split very substantially into factions. Posters here reflect that, and tend to rapidly start denigrating those that don't think like they do. Left wing politics has also developed a fairly weird hagiographic tendency, and this often makes reasoned arguments almost impossible. This, because we all care, and are in a general sense of despair about the way the right is taking the world, tends to raise passions to the point where innuendo, anger and insult take over.
Since the politics forum is a sub-section of a football site, I would take the view that moderators feel it's not worth the candle. Hard enough dealing with the WUMs and depressives on the football threads, let alone a sub-section that probably no-one would miss if it got deleted.
I imagine if we were all better at self-moderation, there would be less locked topics.