Sync had Lemmy.world as the default instance to register a new account (might still be the case, I'm not sure). One of the factors for sure.

Why are people joining .world to begin with?

Because how it works when you first join is very confusing, and why you would choose any particular server is not clear at all.

Also, people want to join something that is bigger and more active because it feels like it would be better (more stable, more content, etc.)

Because they have no basis on which to decide where to go. It's like buying toothpaste but there are hundreds of options, none of which you know anything about, so you get whichever seems most popular. It minimises the risk of ending up with something which is unpopular for good reasons.

If you join a small instance, the chances are higher that it will a) be poorly maintained and b) fold quicker, forcing you to find another instance to join and re-subscribe to all your communities.

Lemmy.world is also notoriously mismanaged and has had dubious privacy issues in the past, such as their Discord situation regarding user messages

Everyone should leave that instance, the admin and the mods on that instance are big time thought police and will find excuses in their vague rules to delete your posts and eventually ban you if your views go against the grain.

Also, don't put all your eggs in one basket: diversify, diversify, diversify. Make a main account, but have more on at least one other instance. Instances go down for maintenance, software gets updated, owners change moderation policies, so on. If you can't get to Lemmy through your main, use your secondary.

Personally I use lemmy.sdf.org as my secondary. It's run by a bunch of retro-enthusiast Unix nerds who more care about the functionality of the tech than anything else. No blocked communities there, and AFAIK they haven't defederated from any instance outside of ones that were hacked/compromised. That does not mean you can just go there and be a shitbird though, they do have standards.

I think it's fine to have a main instance, as long as that main instance isn't gung-ho about censorship like lemmy.world has become.

Although I definitely agree with recommending against joining lemmy.world.

Such a shitty decision by biased mods and useful idiots. They deserve to lose power as a result.

I don't disagree at all but I can kind of understand why a lemmy instance would block piracy communities. Reddit has many millions of dollars and a squad of lawyers to back them up, lemmy admins don't.

Lemmy is confusing enough for people who are not used to its idea. Everybody new and with FOMO immediately went for the bigger instances.

why? because it feels safer to join a big and longer lasting instance than a random one with almost no users. such small instances can vanish from one day to the bext. i once created a account on such a small instance and not even a week later it was wiped from earth, taking my account with it. so it's no wonder people chooser rather bigger instances.