The fact they even felt the need to create another protocol instead of using, improving and contributing to ActivityPub makes me think they want to bake a busines modell for themselves into the technology and they can't do that if they don't have full control over the spec.
ActivityPub is organically grown from a community to serve that community. Bluesky is created to combine making the profits of a commercial centralized social network + profiting from the investment from crypto bros.
ActivityPub is not perfect and nobody is pretending like it is (there is a big blog post from the co-author about what retrospectively they should have done differently, sadly I couldn't find the link), but a specification is not static ! If you have a suggestion to improve it you can propose changes and a lot of things are left to the implementation, so you can just do things differently than other ActivityPub application currently do.
In fact there are people building the future of the ActivityPub ecosystem right now: https://spritely.institute/
There is no need to create yet another specification. The only feature this claims to have, that all ActivityPub implementations currently don't have is identity portability and that can actually be implemented without changes to the ActivityPub spec, just no ActivityPub Application has implemented it yet.