I could be wrong, but I hold the opinion that ATProto is the CDMA (3GPP2) of social media protocols, while ActivityPub is GSM (3GPP).

CDMA had better radio tech than GSM, but at the expense of openness. Qualcomm basically owned CDMA, and still does, while GSM was cross-licensed among everyone.

Likewise, ActivityPub is truly open while ATProto is "open" but you're basically a prisoner to Bluesky Social, the way CDMA put you in Qualcomm's prison.

Bluesky has the initial lead, but it's Twitter's estranged child. People used to Twitter find Bluesky an easier replacement. CDMA was also an easier upgrade from analog 1G networks than GSM was, due to re-using the back office systems and ESN identifiers.

Yes, Bluesky has a better experience. But maybe future ActivityPub releases will catch up for a large part. UMTS caught up to CDMA while being more open, and LTE became the universal 4G standard, with GSM-centric IMEI and SIM cards and such. And maybe PDS implementations will converge to ActivityPub with an ATProto fallback.

Keep in mind that I know nothing about the protocols, so I could be missing what makes ATProto a better tech, or not.