Well it is the environment, also one way of looking for harappans is through their counting system and weights, we also know they used certian forms of numbers.... the sites founf in the south have been seen as trading posts and colonies, I know archaeologists believe in several theories to Harappan decline, invasion, economic and environmental, they think all these factors and not just one drove the civilisation to an end.
We can only speculate and have to conclude that it must be several factors and not just one. Till more is know it will be hard. Problem is the rate of decompisition ad preservation, for some reason the environment does not help in preserving archaeological remains to allow us to discovered what happend.