Essentially, gatekeeping. Places that are hard to access without the knowledge or special software, places that are invite-only, places that need special hardware...
Or places with a terminally uncool reputation. I'm still on Tumblr, and it's actually quite nice these days, mostly because "everyone knows" that Tumblr is passé, so all the clout-chasers, spammers and angry political discoursers abandoned it. It's nice living, under the radar.
Another important factor is whether the place is monetizable. Places where you can't make money are less likely to be infested with AI.
Or a place that can influence a captive audience. Bots have been known to play a part in convincing people of one thing over another via the comments section. No direct money to be made there but shifting opinions can lead to sales, eventually. Or prevent sales for your competitors.