“Hey, I like what we're doing, we should set up a group for it to make it easier to keep up with events and stuff.”
“Telegram? That's what everyone else seems to use.” “Yeah, I've already got that on my phone.” “Me too.” “Yup! And I've got this great sticker pack I want to show you…” “Hey, can you show me how to install it?” “Sure, it's easy, just…”
“Awesome, I'll set it up.” (tap, tap, tap)
(Substitute Facebook, or Discord, or whatever else.)
Gradually people just kinda forget to post things anywhere else, because it's convenient and lets them post from their phone and they get all the encouraging responses they wanted there, and so it gradually becomes common knowledge that that's the place. The choice of platform isn't in their community identity, so the default is whatever people gravitate to that doesn't require doing something unusual.
If you don't integrate psychologically with the popular platforms, you're not really choosing to not be part of a specific community, because that's what “popular” means: you're choosing that the set of communities you can explore in the first place is whittled way down, and your membership in any you do find is now precarious. “Pick a community that doesn't do that” not only raises the difficulty at the beginning, but it also doesn't save you later.