This past weekend's DC Fandome certainly lived up to most expectations when it came to the volume and the quality of the reveals. We even got a decent look at one movie that hasn't been able to begin filming in Black Adam. Through simple animated sequences of concept art, we finally got to see a version of what Dwayne Johnson will look like as the anti-hero of the film and we also learned who he'll be battling in the form of the Justice Society of America. The makeup of this version of the team was confirmed to include Hawkman, Doctor Fate, Cyclone, and Atom Smasher, but fans immediately began asking about another potential member. Hawkgirl.
If Hawkman will be in this film's version of the JSA, then it would make sense that we'd get Hawkgirl as well, and many asked Dwayne Johnson about it on social media, wondering if the character was perhaps being held back. The Rock responded with a somewhat cryptic answer, that confirmed that Hawkgirl actually was planned for the film at the outset, but was ultimately replaced by Cyclone for reasons Johnson calls "complicated."
While Hawkgirl was, like many female heroes, simply created to be a gender-flipped version of a male hero, Hawkgirl has become just as popular, if not more so, than her male counterpart. While the character has an extensive history in the comics, having been around since 1940, non-comic fans may be familiar with Hawkgirl from either Legends of Tomorrow or the Justice League animated series from the early 2000s.
Dwayne Johnson says that down the road the story of what happened with Hawkgirl will be told, but he's not going into it now. LIkely, DC and Dwayne Johnson don't want Hawkgirl to be the story when Black Adam hasn't even gone into production yet. Nobody wants the movie or the character of Cyclone to start off at a disadvantage due to fans who wanted to see Hawkgirl being frustrated.
Of course, not knowing the story will lead to speculation about what exactly happened. Since Warner Bros. owns DC it's not an issue of rights management, so the first thought that pops into mind is that there might be other plans for Hawkgirl at DC and Warner Bros. Because Hawkgirl is a fairly popular character there may be interest in doing something else with the character and thus putting her in Black Adam could make that more difficult.
While there has been less focus on the "DC Extended Universe" as an entity in recent years, there still is one, at least as far as we know. Black Adam was referenced first in the Shazam! movie, which we also know takes place in the same universe as Justice League. If there's a competing desire to use Hawkgirl elsewhere in that universe, then it may be that Black Adam was asked to replace the character, similarly to the way that the first Suicide Squad movie had to be changed up in order for Justice League to work.