I wonder what it is that is keeping more diverse users away?

One aspect is that federation is definitely a bit harder to wrap your head around technically.

But I think another large contributor is the fact that culturally, the zoomers never really grew up with things like independent forums. I’m 33 and back in t the day it was very common for me to be signed up to many different forums for my different interests. Over time, I’ve seen the centralization of those communities, forums shut down and centralized services like Reddit, and lately Discord took their place.
I remember a time when the internet wasn’t solely controlled by a handful of organisations, I can see the value in federated systems. But someone who only knows centralized services and walled gardens is likely to fear the wild, or at least won’t value it as much.