Just lack of numbers. Reddit's at it's best when I can use it to discuss some incredibly niche topic. That early 2000s RTS that nobody remembers? Got a few dozen redditors still posting memes. New indie game drops? There's enough redditors on it that we can talk about it.
But lemmy seems really bad for trying to enjoy any community that isn't a big political or meme centerpiece. Any particular game or IP that isn't a lowest common denominator? It'll get maybe 3 posts a month.
No more interesting discussions of gameplay mechanics or inspirations or character analyses, no burning out an entire workday browsing the top all-time and giggling like an idiot, it's just dead here.
The same massive numbers that made reddit insufferable for some are what make niche communities inhabitable at all.
It's perfectly fine to use both platforms and still enjoy the niche subs on Reddit. It's a shame they're not all in one app/website but it is what it is.
An AoE2 licensed reskin that added new mechanics like shield generators and air units. Still have fond memories of it, and it's been absolutely wild to see that there's still a community for it after all these years.
Numbers are a double edged sword. Yes, Reddit has lots of active niche communities that are active because of its massive user base, but once you get to anything not niche, there are so many users that any sort of constructive discussion is nearly impossible are you're drowned out from the masses. Using Reddit only for the niche stuff (with ads blocked, of course) and Lemmy for anything else seems like a reasonable solution.