The civilizations in South Asia, as well as their major cities, were always located at places that had an abundance of fertile farmland. This usually meant that the major cities were established in close proximity to the large rivers, as their valleys had a lot of fertile farmland.
The main reason why the major cities in this part of the world were built in areas with lot of fertile farmland is because the economy was based on the agriculture. Another reason was the large populations on small areas. So basically, lot of food needed to be produced in order for this societies to be able to survive, and if possible to trade some food for other goods that they didn't had.
