There might be a significant number of users here waiting for everyone else to switch over to lemmy. If you start a niche community, it's a little easier for someone else to be like "It's kind of empty, but it exists on lemmy too." What you need is a critical mass of people. It usually takes time and effort to reach that, and someone must be first.
It didn't happen in one big exodus, no. But maybe in the future someone will find those old posts and decide to make a new post instead of just concluding there's nothing and not doing anything.
If someone wanted to start a community they might actually do something to generate interest, nobody wants to put the effort in to build up a community that the mod can just ban them from or they look and go "Its not that theres nothing, theres just no interest."