infiltration concerns. Massive honeypots can and will be targeted, nothing is more existentially threatening to the state than anarchism, not even fascism or tankie ideologies come close if we're talking about well developed/large/matured groups of people. That's why anarchism is so subculturalized to marginal groups (youth/queer/punk/etc), and constantly portrayed this way to outsiders. and that's why you'll get infiltrators from both far-right and state actors whenever an anarchist space crops up just about anywhere, but particularly online.

lack of purpose. So, is the social glue going to be there? How is the group/meeting place/forum/etc going to serve the needs of the people who are part of it? That's the only way you're going to see people investing their precious remaining time and energy, that isn't sucked off by work or some other form of oppression, into an inherently risky endeavor.

safety. Many anarchists are overcommitted, isolated, have trauma, whatever. This makes for very dysfunctional social communities and spaces. I joke with a friend that anarchists are all in "an abusive relationship with each other" in my town. Even in large cities with sizeable anarchist presences, the communities are still surprisingly human-sized, and subject to many problems owing to traumatized people retraumatizing each other, or just plain sabotaging things intentionally or not. You need to have a process for healing people and dealing with the bad actors, it always seems to happen with particular intensity in anarchist communities and groups.