One other possibility, touched on in part by SG, would be extreme seasonal changes in the climate or a particularly long climate cycle (i.e. the whole Ice Trucker thing that aired on television where they can only run their trucks during the coldest part of winter). But Gidde hit the nail on the head, so to speak. Human cultures didn't "settle" down until after a certain level of agricultural advancement and, in many instance, the ability to handle herd animals.

GW