@lifeofvibe Gonna make it real simple and what it's really all about at every core of most people's hate for anything but the original (regardless if its a snack, food, shirt, school, or anything else) and I might get some backlash for my blunt response but here goes anyway.
It's because it was Harley's first costume and symbolized her at her most "innocent." A lot of fans were raised with BTAS Harley, even me, while they were growing up and have trouble seeing her any other way because of either pure nostalgia clouding their judgement or making the unwise move to put a fictional character in a franchise that always evolves on the same pedestal and emotional respect to a real family member or pet. Making Harley more mature and more fitting to her role as the popular psychopathic moll is the equivalent, to the most hardcore of fans, to making Peter Pan grow up and be an adult and get a job. It's pure blasphemy to these type of fans and it creates the feeling that possibly the only good thing in their childhood is tainted and perverted now or the only thing that was purely "innocent" about Harley is taken away. It's all about what is innocent or pure at the end rather than actually analyzing and criticizing the concept in order to make it a better character.
I could say a lot more about this topic but I think @dr_harlequin covered it more on this topic.