I think the main thing holding back lots of developers from adding ActivityPub support to lots of platforms is the fact that although the AP Spec looks great and seems very good, it is nearly impossible to make much progress with coding because when you try to federate against Mastodon or some other AP site, stuff will not work, and there’s absolutely no way to know what’s going wrong.
Without a test suite tool, the only way to make progress is to get a copy of Pleroma or Mastodon, and install it, run a server yourself, and basically reverse engineer it by watching the HTTP traffic or logging.
Obliviously this is a hurdle not many are going to get over. It’s even worse than that, because both Ruby and Elixir are oddball languages that hardly anyone is using, so the odds are that 99% of the devs that want to implement an AP server are just going to find it nearly impossible.
So the net result is that there is MUCH lower and slower adoption of ActivityPub in the world. Once there’s a test suite available the ecosystem will really take off rapidly, but not until then.
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Update: There’s one other alternative that ‘could’ work instead of a test suite tool, and that is a set of complete “Request and Response” conversations for various example scenarios (AP actors and actions), that contains all the HTTP Request headers, URL parameters, and JSON responses, so that there’s no “guess work” having to be done to interpret the spec.