Answer:
Because of the different rivers.
Step-by-step explanation:
First of all both Egypt and Mesopotamia used the rivers to irrigate the fields, using the river water with a canal system to deliver water to their homes.
The most important difference between irrigation in Egypt and Mesopotamia was because the Nile water that pass trough Egypt was easier to be used, they knew exactly when the water will flood and where the water comes out. On the other hand the rivers in Mesopotamia - Tigris and Euphrates where different in construction and it was not possible to know every movement of them ( canals, floods).