Decentralization and censorship resistance.
How does a small number of relays offer either of those things? Posts are centralised through the relays; and can be censored or even deleted by the relay operators. ActivityPub is wholly decentralised, and is just as resistant to censorship. So is a blog that uses RSS. Why is Nostr better?
With ActivityPub, user's get censored and deplatformed or rugged quite often by server admins. I can publish my content to 100's of relays, and I do. The architecture is just different. ActivityPub has been around since 2016, so if course it has more users, but as people see how a servee admin can rug you but a relay admin cannot, they'll change their minds.
I she to read your comment again as it just doesn't make much sense to me. How can notes be centralized through relays? How? How? If I am posting my notes to 100+ relays and they all store my data, that is the absolute definition of decentralization. With ActivityPub, I store my posts on ONE server. Sure, dozens and dozens maybe even hundreds of severs exist, but using your words, they're centralized through each serverz because of the mast.to server goes away in an instant, all users that use it also lose their data and entire social graph. ActivityPub is more distributed computing and not decentralized computing.