I think most of it has to do with that lemmy.world has better hardware than other instances. The admin Rudd has a lot of experience running federated services as well. So it may be his first rodeo lemmy-wise but not hosting a federated service with a large user-base.
So when a lot of smaller instances started getting overwhelmed and stopping signups, lemmy.world was going strong without the performance issues that other instances might see.
That along with the fact that NSFW content is allowed makes lemmy.world a good alternative for Reddit refugees looking for something stable with a similar set of rules as well.
I myself joined lemmy.ml at first, then beehaw.org when lemmy.ml asked everyone to spread out, and finally found home on lemmy.world because I didn’t really like how downvotes are disabled on beehaw. Not to mention the defederation that beehaw has done recently. Although I can understand and appreciate why they’ve done that.