A well acted part in a Scorsese movie made her a bankable name for getting hired in Hollywood, not some shitty DC movies (the first Suicide Squad was so bad I certainly wasn't going to waste time on any of the others). If she is given a good script and a competent director, she has already shown she can do the job, on top of still being very attractive. Besides, actors being in moves doesn't put asses in seats like it used to.
Batman is DCs most popular IP, so sure, Harley cosplay certainly went into maximum overdrive after the first movie, but her and Ivy were already a popular choice at Comic Cons before that due to her use in the various animated series as well as her ties to Joker, who is the most popular villain in DC.