Reddit cat subs are tightly knit with people suggesting other related subs for some content. Being able to say “aww, cute angry black cat - this would fit on /r/stealthbombers” that sort of interconnection is difficult on Lemmy making the tangent discovery, reposting and niche cat subs more difficult.
Without the fairly consistent cross posting (and thus part of the discovery), such niche subs are going to be harder to grow a community for.
It might be a bit more feasible if there was a “cats and only cats” lemmy instance where all things cats are posted (compare programming.dev and startrek.website which are two single topic instances and browsing communities on each makes targeted discovery more feasible).
The other part is that those niche cat subs are filled with “normal” people. There usage stats show most people used new.reddit.com or the official mobile app - even before the api changes. Most of those people didn’t leave Reddit (with the exception of a few mods where /r/cathostage is restricted but /r/cathostages which was created in response has even more activity).
This also gets to a “there aren’t enough fun things on Lemmy”. The flairs of /r/cathostages are fun.
If you look at /r/leagalcatadvice you will see that most of its activity is from crossposted (not by original poster either) into the sub and then people are having fun there.
The software supporting fun (flairs, easy cross posting) and culture are currently working against Lemmy for niche cat subs. Consider that !cat has 84 users per day and fewer posts per day than /r/oneorangebraincell - it might not be able to support the range of niche cat subs that Reddit is able to.
I’d also point out that Lemmy’s 40,000 active users and 400k total accounts is comparable to the subscriber base of /r/oneorangebraincell that shows 442k accounts… just for pictures of orange cats that share the one brain cell.