How come before Columbus, the North American Indians were nomadic but the South Americans lived in cities?

When the Europeans discovered and explored America in the late 1400s to the early 1600s, they found that North American Indians usually lived in tents and were nomadic. Yet, the Central Americans in Mexico and Guatemala and Honduras, and the South Americans in Peru and Bolivia, lived in cities like Tenochtitlan, Tiahuanaco and Cuzco. How is it that there is a huge difference in the way these people lived even though they were on the same continent (and supposedly from the same ancestors who came from Russia via the Bering Strait into Alaska) ?