The Tigris and Euphrates rivers in Mesopotamia were erratic. The rivers would flood or drought unpredictably and even shift their courses. Also, the Tigris had high banks which meant water irrigated from it would have to be drawn upward from the banks.
The Nile on the other hand followed a consistent annual inundation that brought water and fertile silt deposits right onto the crop fields. Egyptian farmers simply had to wait for the flood to recede some and plant their crops; the water and nutrient soil was already provided by the River.
