Because the decision of which politics is disallowed is a political discussion, and those people will bring their politics of bigotry to the table and argue that “no politics day” means minorities should be silenced.
Uh oh, you got politics in your no politics.
On lemmy I’ve been accused so many times of reverse-racism and reverse-sexism, where advocating for minorities brings out the trolls who want to muddy the waters and claim that noticing bigotry is the real bigotry actually, and they had big vote pile-ons to go with those comments.
This is largely from big instances with open sign-up. I assume lemmy attracts the right wing trolls who tend to get banned from other places, which is one downside of being a viable alternative that isn’t yet mainstream. Also I’m sure they target this place because it is generally left-leaning.