Egypt is the most stable of the 3 civilizations mentioned. Notice how the Egyptians stayed equitable in trade and never got conquered until the Roman Empire annexed it in 30 BCE. India was busy breaking up and integrating over and over. Even now only the strength of the superpowers like China and Russia are keeping them in check. They also have overpopulation issues, too.
China was also breaking up and reintegrating most of the time. notice how since China has it's first dynasty in 2070 BCE, they had 5 dynasties in 2,000 years. Egypt only maybe had one or two, beginning with Narmer uniting Upper and Lower Egypt.
