A multi-community would be all communities with a certain name, across all instances. It would prevent powermods from being a problem on Lemmy. i think it should be notated with m/<insert name here>, just like communities but with m instead of c.
my answer is that the devs have other, more pressing issues to work on but that begs the question: what?
Performance, security, reliability, other quality of life features, better UX, there are 374 issues open on their github: https://github.com/LemmyNet/lemmy/issues
Until recently it was impossible to browse All due to an issue where it would auto-refresh the feed with multiple posts every second. This and similar issues necessitated a big rewrite to move away from websockets.
Then that was fixed, but it was fixed the same week as the Reddit API went down, so making sure everything was stable and stopped setting on fire under the unprecedented load became priority.
All kinds of other things are still going on, for example there are continuing issues with federation not working as expected which is literally the main feature of Lemmy.
Devs have to prioritise, and “nice to have” features might be a way down their list. That is ultimately the answer to the question in your title.
Lemmy is primarily funded by the NLnet foundation and they have a previously defined (previous to the reddit exodus) set of milestone features that unlock funding from the foundation. The devs need to balance new issues that stem from the influx of users with keeping work going on those features so they can keep getting paid. I can’t find it now but one of the devs wrote a good statement on the gap between donations and their foundation funding, long story short, they really need the foundation money coming in to stay afloat.
