In short because agriculture, animal herding and domestication developed there before anywhere else as far as we can find, almost 8,000 years ago.
By 3,000 BCE, the people living there (the Mesopotamians) had already invented the wheel, developed writing, and created the world's first cities and monumental architecture.
It was an ideal place for a civilization to spring up (between two rivers - the Tigris and Euphrates), an extremely fertile area.