Populated niche communities. The Reddit Exodus created a big blip that now, a month later, well it seems to have died down.
A few thousand people gave Lemmy a shot, and after lemmy.world had issues with the traffic, and went down a lot this past thirty days, I think people just stopped trying.
This is the best Reddit alternative out there and still it's got such poor adoption because it's so different under the hood.
I myself feel I'm missing out on so much content here as was available on Reddit, but I'm trying to actually make a difference by not going back.