Having your identity being tied to an instance is not great UX imo. Bans happen, not always for great reasons. Instances die or close up shop. Having to re-do all your subscriptions, losing your comment history, etc just because your smaller instance closes is pretty annoying and pushes people to more stable, centralized, large instances.
This is why I prefer nostr over mastodon, it's basically the same in every key way except that your identity is not tied to your instance. I believe nostr devs are working on a reddit clone like lemmy or kbin, but it's not done yet. Their twitter/mastodon clone is great though.
It's just a different protocol that makes different trade-offs, so it can't use AP protocol. Nostr is an underlying protocol in the same way that AP is, so you can build twitter clones, reddit clones, video streaming services, etc on top of it just like you can with AP.
Nostr's key difference IMO is where your identity lives, and nostr decided not to have it tied to a particular instance. AP decided to have it tied to an instance, that is a pretty fundamental part of the AP protocol and is the same way e-mail works.