What really happened to one of the world’s oldest and most advanced civilization is not yet known. The reason behind the decline of Indus valley civilization is based upon the speculations of historians.
It is believed that the invasion of an Indo-European tribe from central Asia, the Aryans caused the decline of the Indus valley civilization, as they were the next settlers there. Some others opined that an increase in inter-personal violence and infectious diseases like leprosy and tuberculosis led to their decline.
And some speaks of a great drought in the area, which led to the decline of trade with Egypt and Mesopotamia, and caused the collapse of the Indus valley Civilization.