It takes a long time for sites to build up a large userbase, especially when there isn’t much to differentiate it from close competitors. Lemmy from your average user’s perspective is Reddit but with no ads and less content. Measure that against Reddit and your average community is going to struggle to reach the size of its Reddit counterpart. Unless, of course, its Reddit counterpart got the banhammer.

the ui/ux needs work, most people don’t care about privacy, it’s mostly used by tech people and link aggregators in general tend to have very niche userbases