The usual explanation found in history books is that the inhabitants of the Harappan cities were driven out by the invading Aryans.However it is now recognized by scholars that the Aryan invasion theory of India is a myth that owes more to European politics than anything in Indian records or archaeology.
These discoveries should help put an end to all speculation regarding the Aryan invasion as the cause of breakup of the Harappan civilization.
On the other hand we now know that the Vedic civilization far from coming into existence after the Harappan, in fact ended with it; the mature Harappan civilization was the last glow of the Vedic age.
Weiss of Yale University has determined that most of the old world civilization were severely affected by a prolonged drought that began about 2200 B. The drought may have been triggered by massive volcanic eruptions.
According to the findings of this historic study concluded only recently:"At approximately 2,200 B.
The basic point is: as a result of several independent explorations conducted over a vast belt from southern Europe to India, it is now clear that civilizations over a large part of the ancient world were brought to a calamitous end by an abrupt climate change on a global scale.
To attribute a global calamity of such colossal magnitude to nomadic 'Aryan' tribes is simplistic in the extreme.
Elaborate structures like the Great Bath of Mohen-jo-daro, the Lothal harbor or the citadel at Harappa are inconcievable without a detailed knowledge of geometry.
The world had to wait 2000 years more, till the rise of the Roman civilization for sanitation and town planning to reach a comparable level.
Further, there is profuse archaeological evidence including the presence of sacrificial altars that go to show that the Harappans were part of the Vedic aryan fold.