Most likely because they care less about the idea of federated platforms and more about "not Reddit" and "not Twitter." I'm one of those users personally (not that I don't care about the idea, it's good to have a return of what is effectively 3rd places of the internet). Most of them, like me, probably came here during the Reddit migration and moved to BlueSky when that took off in popularity.

If I didn't dislike the Twitter format as much, I'd probably spend more time on BlueSky than forgetting about it until one of these threads appears, and I'd probably be on Tumblr still if I didn't only use social media from my phone and Tumblr didn't have such a horrible app.

People are going to go where the people are, for better or worse, until something pisses them off enough to go somewhere else. I originally created a Twitter account to follow a bunch of artists I followed who left Tumblr during the porn ban. I didn't care for the platform (I hate the tweet format) but that was where all the artists went so I followed. Similarly, when the 3rd party api fiasco hit Reddit, I left and immediately went looking for where the people from the subs I read by "newest posts first" went - except the communities fractured and disappeared. It was the possibility of them reforming here that made me go through a GitHub to figure out how to make an account (spoiler: they never really did reform). I had no idea what a federated platform was supposed to be or do.

The fact that Lemmy is so niche is its biggest advantage and its biggest curse. You either love how small it is, like Reddit back in the day, or you suffer the lack of population for the things that you're into, and the very nature of the federated platform makes it that much harder to centralize enough people in one niche to form a community (there we go again - centralization). Lemmy is the Wild West frontier town to the big social media giants' company towns.