Network effect. The communities and discussions are still there. Lemmy has the infrastructure, mostly, but not the users and not the discussions at least not in anywhere near the same volume.

There's also a few seriously missing features with Lemmy, for example modmail.

I have also found, overall, as stupid and crazy as this may be, when discussing anything involving politics users on Lemmy can be almost militantly closed-minded. I find more intelligent discussions on Lemmy, but I also find more of the 'you either agree with me or you're everything that's wrong with the world' type.