At each WorldCon, host committees of science fiction groups from cities put up bids to host the WorldCon to be held two years hence. In 2021, Chengdu and Manitoba had committees on the roster. WorldCon voters chose Chengdu.

The problem is one of effective power: do you tell people who live in these regimes, "sorry, you cannot participate in this culture because your government is bad." How much pull do they have to change things, especially this sort of thing? Or do you say "you can participate, but only at a lower tier." Either way is going to put some people into an out-group. Maybe that's the right thing to do--paradox of tolerance, for instance--for the rest of the people in the group or culture. Maybe it isn't.

I know it would hurt pretty badly to be a science fiction enthusiast in China, perhaps even someone who really does not like what their government does, and not be able to participate. On the other hand, it hurts to be potentially on the list for your work's most prestigious honor, only to be "disqualified" because you "might" have written about something that isn't permitted in the host country you couldn't have predicted.