(2/?)
Friendica designed their own protocol, DFRN, that incorporated private sharing. Then later added support for OStatus, within the limits of what could be federated using it, and after Mastodon and others started using it, ActivityPub.
But by the time they did, they were in a network dominated by Titter leavers on Mastodon services. Plus GNU socialites who were used to a public-only network and distrusted Mastodon's Direct posts for private messaging. Thus the bias towards public posting.