I cant believe I forgot something so obvious. The reason she wears that uniform is because she used to be a psychiatrist. Silly me.
Also, I think the oldschool batman outfit doesnt work well in the realistic setting (underwear on the outside has never been a good idea), but I doubt they thought they could have gotten away with changing Batman without a scratch.
and Harley's always been in the "OMG she's hawt!" camp, no matter how much she covers.