First, the importance of, for example, email has steadily increased over the decades, and thus email has become something we expect to be faster and faster.
Second, as consumerism demands more and more faster and faster, the American culture (and many others) has faithfully obeyed, speeding up everything it possibly can.
It’s easy to imagine that if a culture were completely free of consumerism, many none face-to-face communications would be very slow, very async.