This borders on gossipy speculation, so take it with a large grain of salt, but I've heard from a couple supposed industry "insiders" versions of this same basic account: that Robbie, as producer, was presented by WB the option of either a Birds of Prey story or a Gotham City Sirens story, and Robbie didn't want Harley to be "upstaged" by even more popular characters like Catwoman and Poison Ivy. So she preferred BOP, which would feature characters considerably secondary to her.
I don't know that I necessarily believe that, but it's a somewhat plausible explanation.
I'm a fan of Black Mask, as well, and even though I really like McGregor, this much more broad take on the character kind of soured my interest in seeing the movie.