Well for what it’s worth, my opinion, just an opinion, is that the platform was held back by engineering decisions that came out of a web server world, where everything was focused on servers instead of users.
If you look at the technologies underneath ActivityPub, it looks like developers grabbed a bunch of off-the-shelf web technologies and cobbled them together. Lots of http and webfinger and web certificates, etc.
They could have started more from scratch, but this is the direction they went, and for better or worse, it’s going to be a server oriented platform because it was built on server oriented technologies.
I think it was a case of having a hammer and everything looking like a nail 🙂