@bitbearAsbjørn Ulsberg 
@scottjenson I think there were two main reasons. First, Mastodon already had an API when they adopted ActivityPub. Second, the ActivityPub API didn't give them everything they needed for a full client experience.
@bitbearAsbjørn Ulsberg @scottjenson like AtomPub, ActivityPub is kind of a light, minimal standard API. It doesn't include features like search, push notifications, and so on. Part of the work of the ActivityPub API task force is to extend the API with some extra features like those.
If you have a fediverse account, you can quote this note from your own instance. Search https://cosocial.ca/users/evan/statuses/116006484334709333 on your instance and quote it. (Note that quoting is not supported in Mastodon.)