I would say thats partly on the creator favoritism Harley gets. She is the character "everyone wants to write" so if given the opportunity they'll put her in their book over anyone else.
The lack of Deadshot and Boomerang is a good indication of this actually. Both ostensibly popular characters, but they undoubtably do not have the fervent adoration Harley gets by DC's editorial and writers. Harley gets shoved with major plot significance into every AU comic from Dark Knights of Steel to DC vs Vampires while Deadshot and Boomerang are lucky to just get a cameo.