That's become my answer to all "why not ActivityPub?" questions.
replyAP isn't completely stagnant but there's a reason AT is still holding on to and accelerating that early developer excitement AP had. Maybe it's marketing, maybe it's money, maybe it's some technical thing. Maybe it's the community. Whatever it is, people seem to enjoy developing in the Atmosphere in a way I never saw on AP.
If you consider people's worries of centralization happening with AT, that's not necessarily a good sign.
replyOf course VC funding and startup mindsets prefer a protocol that is easier to rugpull.