I think it's the allure of that finely shaped ass that is hiding behind that layer of clothing. The knowing that behind that layer of cloth is a round peachy heaven is what men desire, no man can resist the booty.
Probably because it smooths over any flaws like cellulite, and the top band can help accentuate the waist to booty ratio. Not to mention the "je ne sais quoi" that comes of leaving things just slightly up to the imagination.