Answer :
Because the area or region they were orginally in had been destroyed, they probably were more cautious and careful of the resources that they used in these new regions.
Climate and geography had always had a big impact in how people lived their lifes. In the case of the Sahara, what we have observed is that the population that lived there were forced to migrate and found new ways of living. Some of them found new fertile territories and stabilized there, others become or remain nomades, and they continued to be nomad even these day. Beside form that, changes in diet where of course made, provoking a cascade of changes in rituals, culture and societies norms.