Mesopotamia is incredibly fertile, and this area of the world is the birthplace of agriculture. Without farming (and not just eking out an existence, but farming that produces a surplus), you don't get urban settlements or the stratification of society into distinct social classes. You basically had the richest and most technologically advanced societies on the planet be able to exert their influence over larger areas than ever before in history.

Even in Roman times IIRC the majority of grain in the empire was produced in Egypt. They don't call it the "fertile crescent" for nothing.

Here is a resource about the emergence of the first cities in Mesopotamia.

Mesopotamia makes up part of the ‘fertile crescent’, a fairly narrow strip of rich, high-yield, agricultural land, surrounded by desert and formed by alluvial deposits from the Tigris, Euphrates and Nile river systems. During the Neolithic, when man shifted from a hunter-gatherer to an agrarian style existence, the fertile crescent became the focus of intense and highly localised activity. Farming requires team work, and unlike somewhere like Europe, where communities would have been far more spread out, Mesopotamia had de-facto, geographic limits placed on where people could and could not farm. Such a situation necessarily leads to closer integration between population groups, thus forming larger societies. Larger societies, in turn, lead to innovations and eventually civilizations…

For a good (and highly readable) introduction to these processes see: Steven Mithen, After the Ice: A Global Human History, 20,000 – 5000 BC (London, 2003)

Also good is: Gwendolyn Leick, Mesopotamia: The Invention of the City (London, 2001)

Georges Roux, Ancient Iraq (London: 1992), also gives a highly informative, if rather dry, account of the archaeology and cultures of ancient Mesopotamia.