Here is an excerpt from the offical docs for the curious:

"Why not use ActivityPub?

ActivityPub is a federated social networking technology popularized by Mastodon.

Account portability is a major reason why we chose to build a separate protocol. We consider portability to be crucial because it protects users from sudden bans, server shutdowns, and policy disagreements. Our solution for portability requires both signed data repositories and DIDs, neither of which are easy to retrofit into ActivityPub. The migration tools for ActivityPub are comparatively limited; they require the original server to provide a redirect and cannot migrate the user's previous data.

Another major reason is scalability. ActivityPub depends heavily on delivering messages between a wide network of small-to-medium sized nodes, which can cause individual nodes to be flooded with traffic and generally struggles to provide global views of activity. The AT Protocol uses aggregating applications to merge activity from the users' hosts, reducing the overall traffic and dramatically reducing the load on individual hosts.

Other smaller differences include: a different viewpoint about how schemas should be handled, a preference for domain usernames over AP's double-@ email usernames, and the goal of having large scale search and algorithmic feeds."

Relevant post by Christine Lemmer-Webber (Co-creator of ActivityPub) https://dustycloud.org/blog/how-decentralized-is-bluesky/