I think that for Lemmy and Linux the problem is actually in the people using it. Without people using it, they won't be many posts/good software support. Without the posts/support there won't be many users. It's not some UI being different, or anything else. It's the main issue. When you see lemmy.world frontpage (All, not Local), there are 15 threads about Reddit and Lemmy, 2 about Twitter limits and the rest is about tech. Meanwhile, on r/popular you have variety of communities, still mostly memes and videos but there are also other posts.
It's more about reddit that reddit is. You can't make it long-term with this type of content. On other instances it's more bearable, but it's still not enough to keep people here.