It's not really that deep - it's a combination of network effect and inertia.
Because there's "nobody" on Lemmy, people don't join Lemmy. Because nobody is joining Lemmy, there's "nobody" on Lemmy, and so on and so forth.
Furthermore, Reddit (and other big social medias) already have "everyone" there, so to the vast majority of people they're just superior platforms as it's easier to find both people and content. Moving to another platform means giving that up.
I agree with this. I love the fediverse and have been using it in some form for quite a long time. I've checked in on lemmy periodically over the last year or so, but ultimately I spent a lot of time reading topics on somewhat niece subjects and so the lack of posts has been a huge factor.
It feels like it takes one of these self-imploding events to really cause people to move as one to something else.