Paul Frazee ( @pfrazee.com ):
Bluesky didn't use ActivityPub for 2 reasons —
№1:
Because of the identity issue.
Once you make a choice (of server) you are locked in.
You cannot migrate freely.
(Current Fediverse migration not good enough.)
№2:
The culture on the Fediverse at the time.
There was a culture on the Fediverse of being hostile towards global aggregators.
( @reiver note: AFAICT, this is a very small minority of hyper hostile individuals.)