I can't speak for any specific instance, but I believe many of the smaller ones that defederated lemmy.world were concerned that a large user base with looser sign-up requirements would disrupt the community they wanted to build.
This is mostly correct, but another point to consider (once OP becomes more familiar with the basics) is the concept of defederation. Basically, not all instances permit interactions with each other. Many smaller instances have defederated from lemmy.world, for example. That means that users on those instances can not directly interact with communities hosted on lemmy.world, although you could still encounter users from that instance in a post on a community in a different instance that is federated with both.
If the instance you are currently on has defederated from another one that you would like to browse, you will need to create a new user account on that instance (or one that hasn't defederated from it).
Technically, I didn't buy this, but I feel like it fits the spirit of the thread.
When I was a kid, a friend of mine gifted me an off-brand Super Nintendo controller to me for my birthday. I used it for all of about 5 minutes before it shocked the shit out of my hand and then never worked again.
I have a government-issued iPhone. It has Signal on it, and I have access to the appstore. It's also not allowed to be used for anything but unclassified communication and isn't permitted inside restricted areas.
Let's also be clear: Signal, regardless of their encryption standards, is not an approved system for any kind of classified information. Leaks of this nature have the potential to cost people's lives. Every single person in that group chat would have known this. Many of them have original classification authority.
Further, not only was the platform not approved for the information, the messages were set to disappear after some time. This is a violation of government record keeping laws and FOIA standards. This wasn't an oopsie.
And now I have a phone capable of taking photos too large to be stored on that drive. Crazy how quickly technology can progress.