Not an expert, but it boils down to two main factors for me:

The biggest factor is the nature of the userbase that migrated. As with Voat, there are (i) people driven here by ideological means ("it doesn't affect me but I believe the change is wrong"), and (ii) people who have no other option ("I need a new home because the change makes reddit less useful for me"). Of all the folks who belong to (i), i wouldn't be surprised if majority move back to reddit quietly over the next 2 months. The reality is that people are used to reddit, and it contains a decade's worth of information and social networks that lemmy doesn't have. It's simply easier to find info on reddit. That leaves (ii), which will probably make up the majority of long-term migrants. Unfortunately for Voat, (ii) was comprised of people who post/read actively on hate subs. /v/fatpeoplehate was one of the top, er... subs? at Voat. When your userbase is gathered around one specific, kinda-toxic purpose, it's hard to see it succeed. For lemmy, (ii) consists of people with strong preferences for third-party mobile apps. I don't know for sure, but I think these folks are (like me) long-time redditors, power users, and generally contribute more than usual. Third-party users are also more diverse, and don't have one specific interest. That is a more self-sustaining community.

A smaller factor is the nature of the site. Voat was basically reddit, with less moderation. Literally no other improvements. The fediverse is interesting, and I think it's better. Even if you disagree, it's still objectively different. A relatively small site can't succeed against a behemoth if it's exactly the same.