The most interesting theory is that a massive earthquake changed the course of the rivers so much that a lot of the tributaries dried up. Without adequate water supplies for irrigation, the cities couldn't sustain themselves, so people literally just picked up and left for greener pastures. Well, probably not pastures. It's unlikely they became nomads. They probably just moved to a different plain and continued their agricultural ways. I am already boring you and I haven't even told you yet how to be a better boy and or girlfriend. I'm going to do that now.