No shirt, No shoes, No Rump Shaker Coverings, No Service. I assume it just sounded better. Skirts, dresses, kilts, pants, shorts.

Quick search found this: “No Shirt, No Shoes, No Service” was a response by businesses in the 1960s and 1970s to keep long-haired hippies out of stores and restaurants. There are no federal or state laws to this effect. However, there are laws that allow businesses to make their own regulations. This phrase has become an accepted norm