part of the reason why some communities stick and some dont is because the type of people who were willing to jump to lemmy. Users who jumped to lemmy generally were more tech forward, privacy forward, or was part of some ostracized community not very welcome on reddit, as the typical casual user does not care for site politics.
its really hard to start a small community with one main poster, and requires a few to get the ball rolling. its a game of converting those in the niche communities to make the jump