I suppose the onboarding has a lot to do with it. Creating a mastodon-compatible account is a hassle. You have to ke choices. The experience isn't seamless.

No, it's mastodon but centralized. It takes all the difficulty out of signing up for the fediverse, like finding a server. I said it from day 1 on mastodon. We will never see mass adoption until there's a simple sign up process. People like centralized because it's easier.