My guess: They were created expecting an in rush of users and content from R/ but when that didn't happen, they got bored and moved on.
There are many, many, r/sub clones that were created during the API shit. The people creating the subs had no intention of creating content, they just wanted to be a mod. Now they lie as empty as the day they were created.
I don't know how many other people had the same thought, but I thought about snagging a couple community names, not because I have any interest in being a mod (I really don't at all,) but to hold onto them until either former reddit mods showed up or at least someone else who seemed like they had real interest in actually running the place and didn't seem to be an opportunistic power-tripping asshole who would run it into the ground.
I didn't end up doing it, but I gave it some serious consideration.
They are afraid that most of lemmy users will be on lemmy.world and want people to join diverse small instances. The other reason is the amount of attacks lemmy.world got
We're on the fediverse, you can create a community with the same name on another instance.
Most instances are well federated. So if it's not on Lemmy.world it can be on feddit.de or sh.itjust.works it still works, and considering .world uptime issues, I would advise any other lemmy instance or even a Kbin instance to a newcomer
honestly even if .world had 100% uptime i'd still recommend spreading out to smaller instances. it's a single point of failure for the entire network (or at least the parts of the network as seen by lemmy/kbin). bus factor and all
i personally hope that over time as the tooling improves (and as "turnkey" hosters like masto.host adopt lemmy), more topic-specific instances get started up. think of startrek.website or programming.dev or slrpnk or rblind or literature.cafe or all the country instances. and general purpose instances like .world and lemm.ee and sijw and what have you can be for smaller communities that don't have the resources, or they can be used as "account holder" instances that have people but "outsource" the communities to their own respective instances to keep their own costs down.
everyone piling up on .world is hurting the entire network. (even worse when they create communities and those communities also pile up on .world) and it's sad to see the .world admins refuse to acknowledge this and handwave it behind "well we're still smaller than mastodon.social /shrug" (and yes i have complaints about eugen's handling of mastodon.social/.online too)
If someone staked a claim by starting a community, making it mod posts only and then going inactive, all thet need to do is contact the admins of that instance. As long as it's not risky to reopen (some can give life and death advice and need careful modding) the admin can open it and make them a mod.
If it's open and free to post then they can just start posting to it. They can contact the mod about possibly helping out and, if there's no sign of activity, they can contact the instance admins.
Lemmy.World was kind of the go to instance for exiting redditers during the mass exit over the API changes, while a majority of subreddits protested. As we all know, most of the subs did not stay closed. So a majority gave up