I belive that puting opposite factions into the same "cage" makes it harder for them to escape.
Also my darwinist theory for the tribal culture of southern states (read M E and Africa nor Texas and Alabama) that lack of basic resourses like water makes survival of your Family and larger tribe a top priority over coexistance with others with superficial differenses in religion, customs and cultures devised by patriarches to impose a farther "Iron Curtan"
Surviving in the harsher northern climats takes a larger group of people to geather nessesary resurse that are plenty.