The network effect. It's big enough that small forums get enough posts to stay active which keeps more people using it.

For example Lemmy has a 3d printer forum that has a few posts a week. Reddit has forums not just for 3d printing but for every specific model of printer and each gets a much activity as Lemmy's generic forum.

If I'm searching for something, Google will show Reddit content but not Lemmy because there isn't an answer on Lemmy.