The most salient aspect of the geography of the Fertile Crescent is the fact that it was a flat plain with rivers running through it. Both of these helped to make the area relatively easy to conquer and to unify.
Areas that are hard to conquer and unify tend to be divided up geographically. They may have mountain ranges or jungles that make travel difficult. This makes it hard to move armies around to conquer and it makes it difficult to administer the whole are even if you can conquer it. The Fertile Crescent did not have these problems.
Instead, the Fertile Crescent was easy to access. The flat plains and the rivers made travel relatively easy. For this reason, armies were able to come into the area easily and conquer. Once this was done it was not too hard for them to unify the area because they could move around it easily.