What allowed native americas in mexico to settle cities larger than those to area in the north

One of the main factors that allowed Native Americans in Mexico to settle cities larger than those in areas to the north was that Central America contained far more gold and riches that acted as currency and boosted the economy. Practically no gold was found in what is now the eastern United States. 

the development of cities that were built by an earlier civilization

Both countries had to aprove annexation.