Because Lemmy top topic before are only talking about Reddit, and people love talk about it since api contro. Now since Reddit hype are down, people start to think how to make Lemmy topic different and somewhat great than on Reddit I think.. So, it takes time to see the average Lemmy users activity..
If they all came from Instagram, or is used to how Instagram is, I'm not suprised this happens. People use Insta for likes only, not comments/discussions. Sadly.