@_elena I am not an expert but if I can help let me know. I have strong suspicions about why W selected atproto instead of activitypub.
Basically there is more power in the biggest actors, a more "centralized" control, to ban/shadow-ban/censure and pull the plug.
In other words it is more impactful when bluesky sidebanned someone or some community than if mastodon.social would do it. The firehose/relay is a the biggest point of control.