>why not just AT?
Because it's not an open protocol? Technical features are not the only thing that makes protocol better or worse, what also matters is who drives the development and who controls the infrastructure. Today, ActivityPub is the only truly open social web protocol. There's no company behind it, no official app, and no associated cryptocurrency. It doesn't even have a single organization that dictates standards.
BS, in comparison, is not much different from Twitter and Facebook. Sure, a company with a lot of money can win in a competition against the fediverse community, but I don't think it's a desirable outcome.
Why not improve ActivityPub instead?