I agree the UX challenge is much more challenging for decentralised services. I don't know enough about Bluesky to really comment on whether it is centralised or not.

Regardless, I think there's another thing that helped Bluesky: VC capital. In particular, to hire people to work on UX. It's a bit of a pet-peeve of mine, but I find it strange that designer don't contribute more to projects like Mastodon, which definitely need it. Even from the selfish angle of building a portfolio, helping solve Mastodon's UX challenges is much more impressive and realistic, than doing the millionth redesign of Gmail that will never get implemented.

It's a "distributed protocol" but there's really only a single server using it.