If I had to make a wild stab. I would say its because WW and CM are tailor made and measured to always do the right and preach the proper lesson as deemed bye execs. Even at WW's height in popularity, that was only due to her tits nearly spilling out all over the place on prime time tv pre internet.
CM may have had a chance to be popular, but they fucking nuked any personality out of her before her first movie. So no more flaws, no more blood thirst, not even her hang ups with the bad guys. Just by the number can do no wrong woman of power.
Harley is freedom, sexy, and capable. Plays by nobody's rules and makes the hyper serious look stupid while having hot chicks crave her. Joker at this point is a small boost as well.