ActivityPub was developed by W3C, and it's properly decentralized. For Atproto, the concern is that Bluesky will exert control over the protocol once shareholders and profit get in the way of making a good product.

Last I heard they were discussing potentially moving the necessary registry/directory to a separate non-profit "like ICANN", but even with that they seemed noncommittal about it.

I'd love to see more diversity in the federated space. Competition and iterative development is how we make things better. But I need Bluesky to take those necessary steps before I feel comfortable endorsing it over ActivityPub