The New Kingdom began to decline around 1200 BCE, but the event that ultimately brought the era to a close was when the Kush conquered Egypt.
After that event, the long reign of Egypt, over half of the world, suddenly ended.
The Kush civilization developed beyond the reach of the Pharaoh's administrators, just past the point where the first rapids of the Nile River made communication by boat difficult.
The Kush and Egypt had a complicated relationship. While the Egyptians were much stronger as a result of their history, they were jealous of the wealth of the Kush.