[SmG Shy]

No specifics were given by the maker, we are just told that their tongue structure allows the coffee to go down easy.

Other than that one factor can be since Coffee beans are used to cleanse the aroma of existing perfumes for the customers, applying this fact to the ghouls who have strong senses especially the sense of smell, coffee might be kinda like refreshment after having gone through the last nights stinky human they ate for breakfast.