I agree with most people that the easiest explanation for the somewhat underwhelming numbers is that Harley Quinn's popularity coming of Suicide Squad was overestimated and it looked too much like Suicide Squad and marketed focused soley on her. Ultimately it's still good numbers because the budget was reasonable but i dont expect anymore harley quinn movies for awhile.
That's true in general for american comedies/dramas with non-black leads too. It's a myth that movies with a black lead don't sell. Will Smith, Eddie Murphy, Denzel Washington are proof of that..