According to the producers of Birds Of Prey, there were two factors that led to the decision: the fact that the character was being used in a different film that was being simultaneously developed by the studio, and the simple selection process used by writer Christina Hodson and star/producer Margot Robbie in building the list of misfits that would become the feature’s ensemble.
So why did things turn out the way that they did?
The Birds of Prey in the comics actually originated as an alliance between Oracle and Black Canary, with Huntress coming aboard the team a few years later, so the fact that Barbara isn’t included as part of the film has been viewed by some as a head-scratcher.