Because there was less land to farm in kush than in Egypt

Kushites were more motivated to trade
Many kushites moved to Egypt
Kushites became accomplished sailors on the Nile
Kushites sold their surplus Grain to Egypt

and other surrounding regions in exchange for goods and resources that were not readily available in Kush. They also traded valuable resources such as gold, ivory, and slaves. This trade network allowed the Kushites to acquire luxury items and exotic goods from distant lands, enhancing their quality of life and cultural experiences. Additionally, it helped to establish diplomatic relationships and alliances with neighboring kingdoms, which further contributed to their economic and political power in the region.