One reason is because I think other protocols have some advantages. AT is better end user ease of use wise, and plans to let you control your account via a keypair (already possible with your own PDS). Nostr is more heavily decentralized and considerably more flexible than the other two. That can siphon off existing users or have new users drawn to those spaces. Not to say that ActivityPub doesn't also have its own advantages too, but everybody has different preferences and there's now more choice.

There's also some Activity Pub specific toxicity issues. Too aggressive defederation leads to a point where you can't communicate with most people, and there's some opinions in the space that have turned some people away.

But of course things go up and down, and are never a strait line. I'm guessing all three big protocols will continue to grow, and as they get more interconnected everybody wins, and even if Activity Pub has hit a slump the ecosystem of people you can talk to using it has grown 10x+.

Outside if summarizing my previous takes, there have been some new(ish) things I've seen that don't quite sit right. Things from the top down like the social web director refusing to go to conferences that people from other protocols will be present and encouraging people to not even talk about other protocols. Or - anicdotally - seeing random users happy that the influxes are going to others because they don't want 'normies' on Activity Pub or declaring anybody still using Twitter/X a Nazi sympathizer if not an outright Nazi. If the Activity Pub scene is getting really protectionist it could start also having a negative effect.

Again, overall I expect it to continue trending upwards, and there's a plethora of factors that are unrelated to anything negative regarding Activity Pub's community, but the above (and previous two posts) are the stuff I figured worth bringing up and potentially factors in why ActivityPub has seen weaker adoption compared to the other two big ones more recently.