Aristocrats as a class were originally defined by owning land or other sources of wealth. Thus, they have the wealth to travel, and so Aristocracy gives you the Aristocratic Noble, the first Diplomacy unit in the game.
Aristocracy means, in Greek, "rule by the best qualified (people)". The catch is, of course, that the aristocrats usually define "best qualified". Aristocrats in the ancient world were almost uniformly rich landowners, land being the ultimate source of wealth. Their similar lifestyle made them more familiar with aristocrats from other countries and cities than with their own non-aristocratic fellow countrymen. The concept of "guest friends", or aristocratic families in foreign places allied with your own, increased the tendency of the aristocratic class to be international in scope. The great Empires of the Middle East, starting with the Assyrians and continuing with the Neo-Babylonian, Median, and Persian Empires, all used aristocrats from the same economic and social class as the ruling families to rule their Empires. The aristocrat who was, after all, the only individual wealthy enough to be able to travel for pleasure, became the most common envoy and representative between governments of all sizes, just as he was the most common representative of the central government, as a local administrator or ruler, to his own fellow-countrymen.