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.

//edit: Another thing to keep in mind, is that it’s just very common for this demographic to be early adopters for tech products and platforms. I remember when Twitter started, and a large part of its early user base was people in their 30s or older who were very into tech, or journalists. The reason I started using Twitter towards the end of the 2000s was because most of the podcast hosts and regular contributors on the TWiT network were using it.
Seems to me that if you want to launch a social media platform, your early adopters are either guys who are into tech and in their 30s and 40s or teenage girls.