World was already the biggest by far when I first started lurking back in July, and it's just getting more dominant. Before, there was quite some diversity in the distribution of generic communities, but nowadays the vast majority of posts that reach the top are from over there.
I really can't see any specific virtue that it has; uptime is not the best (or so I've heard), the moderation is quite lacking (which is demonstrated by the fact that Beehaw defederated them), they make some unpopular moderation choices (like blocking !piracy@lemmy.dbzer0.com), and overall the atmosphere is a lot less... nice than those of smaller instances.
I also feel like it goes against the idea of the Fediverse that one instance has control over most of the platform. Especially on Lemmy, where communities mean that building community within an instance makes so much more sense than elsewhere, and upvotes are federated near perfectly regardless the size of your instance, decentralisation makes a lot of sense. It really just doesn't make sense to me that Lemmy World is where people are going.
I thought it was because lemmy.ml closed registrations?
My original account is on lemmy.ml, but then I made this one on hexbear.net which now I use as my main, and I remember lemmy.ml was open long enough so I was able to create mine right before majority left reddit and then they were closed as it really spiked. I'm not sure if lemmy.ml registrations are open again but when most people were seeking to join Lemmy and lemmy.ml was closed then it seemed like lemmy.world was the default by default.