Possibly an oversight--there are a lot of cow-print items, but not all of them are part of a complete set. It might just be that the bikini bottom was just made as a stylistic choice (skimpy styled lower-garments). Swimwear flagged clothing was added around the time when the Tarkus pool was implemented, and if I'm remembering correctly, the bikini bottoms predates the pool content, so a bikini top probably wasn't considered at the time.