What are theories about why early cities in the Indus Valley were abondoned?

It was becuase there was a big drought and no food or water.

One theory suggested that a nomadic, Indo-European tribe, called the Aryans, invaded and conquered the Indus Valley Civilization. Many scholars now believe the collapse of the Indus Valley Civilization was caused by climate change. The eastward shift of monsoons may have reduced the water supply, forcing the Harappans of the Indus River Valley to migrate and establish smaller villages and isolated farms. These small communities could not produce the agricultural surpluses needed to support cities, which where then abandoned.

One theory suggested that a nomadic, Indo-European tribe, called the Aryans, invaded and conquered the Indus Valley Civilization. Many scholars now believe the collapse of the Indus Valley Civilization was caused by climate change. The eastward shift of monsoons may have reduced the water supply, forcing the Harappans of the Indus River Valley to migrate and establish smaller villages and isolated farms. These small communities could not produce the agricultural surpluses needed to support cities, which where then abandoned.