I think theyre still popular on Usenet.
IDK all the history but rar has built in ability to create recovery data / parity volumes.
Parity data is like additional data that can help reconstruct any degradation.
That said, usually a standalone parity generator is used which can work with other types of archives, but rar is what everyone uses so why change.
Compression algos are ineffective on encoded / compressed media anyway.
It used to be important on Usenet, and maybe still is, because if a drive starts to fail somewhere and contains errors those errors can be reproduced across the network. Not sure if thats still a thing or why but certainly 10 years ago it was.
The summary to this rambling comment is: some communities still like rar because its what they've always used and there's no benefit to adopting 7z.
Nah, this can lead to having many sparsely populated communities. It can discourage new users, and fediverse adoption generally.
For example, if you look in one of the several audiobooks communities you would conclude that the fediverse is a ghost town. However, the actual "community" is in /c/books. If you post about audiobooks in both you won't get any engagement in the audiobooks communities.
After the great reddit API influx a few years ago a bunch of communities sprang up but were quickly abandoned, contributing to this problem.
There's actually a community discussing these dead communities and how to go about merging / closing the dead ones, but I can't remember what its called.
Of course in some cases with larger communities it might make sense to split communities into different topics, but there's very few communities large enough for that to be genuinely warranted.