I believe the answer is: Nile River, savannas, and mild climate zones provided great features for farming and herding
The are near the nile river contain high amount of water source and soils that are rich in nutrients, which is perfect for farming, and the savannas contain a lot of grass that become a natural food for the herding animals. Mild climate allows humans, plants, and animals to have high longevity to live in that area.