@Gargron @julian i can't really speak for anyone here as a relative outsider, but as someone that has worked with both protocols i think one big reason is the lack of a public, accessible, synchronous space for discussion, development, and support
the unofficial discord linked in that slide tends to be fairly active - i've managed to receive answers to most of my atproto-related questions there, and as a result i've been able to get stuff done relatively quickly. whenever i've had issues with activitypub projects in the past, i've just sat on them for long periods of time because i wasn't sure where to ask about them (and i never really got much response from posts i made on the network about them either)
i see socialhub as a great place for asynchronous, more formal happenings, but there's not much for more casual happenings right now afaict (though i'd love to be proven wrong in this regard!)