Well, they didn't entirely disappear. There were still aqueducts maintained in Spain, for example, until modern times. But as a general rule, these features of Roman civilization were distinctively urban. Villas certainly had baths, but the Roman bath we usually think of was something hat existed in a city. And, as another general rule, the most striking social change accompanying the fall of the Empire in the West was a process of deurbanization. So in essence, these fell out if fashion due to a larger process of social change.