Why were Native Americans much more under developed then their neighbors in the south.
Copyright Skubasteve834, source: Wikimedia Commons, usage via CC-BY-SA.
It's not quite fair to classify native North Americans as underdeveloped. The above image is of Monks Mound, a part of the Cahokia Mounds complex in southern Illinois. This mound was constructed over the period 900 to 1100 CE by a Mississippian culture. This cite is within the Eastern Agricultural Complex of North America, one of the ten or so cultures that independently developed agriculture several thousands of years ago. What is fair is that the Mississippian culture and its predecessors developed later than their counterparts in Mesoamerican and South American.
Their development was late and slow in part because they didn't have as good a base of plant life from which they could develop their agriculture as did their counterparts elsewhere. The plants domesticated in North America (marshelder, chenopod, North American squash, and sunflower) weren't as high quality as the beans, maize, and central American squash developed in Mesoamerica. Monks Mound was built shortly after the Mississippian culture had adopted key agricultural technologies that were developed in Mesoamerica.
Bruce Smith, "Eastern North America as an independent center of plant domestication," Proceedings of the National Academy of Sciences, 103.33 (2006): 12223-12228.