While not a catch all for absolutely every factor, the simple statement that they didn't have enough time really does address many of the reasons why.

Cities, and by extension civilizations, take time, innovation, energy, resources, and luck to build. For a variety of reasons, Mesoamerica was the first region to develop an urban civilization in the Americas. This process began around 3000 BCE in some regions and was underway in all of southern Mexico and Central America by about 1000 BCE [1][2]. It takes time for the knowledge of agriculture, ceramic production, urban life, and all the other aspects of a civilization to spread from one region to another. More than simply knowing about it, you also need to develop specialists and a desire in your own region to replicate, duplicate, or otherwise take up and work with the ideas from a foreign complex society.

By about the 7th century CE, Cahokia was founded in what is now the US State of Illinois. It didn't really reach urban density and size until about the 12th century, and by then it was the largest site in North America. Cahokia is about 2,500km from the epicentre of Mesoamerican urban life, so it took roughly 4000 years for cities to travel those 2,500km and develop in the region. Regions further north, such as the St. Lawrence River Valley (roughly 3,800km from Mesoamerica), were just starting to expand and develop permanent settlements when European contact occurred [3].

To give a European example, the first cities developed in Mesopotamia between roughly 6500 - 5300 BCE (Tell el-'Oueili and Eridu). All cities in Europe are in some way or another dependant upon this development. If you take a peripheral region of Europe, such as Britain, you don't get the first city until the Roman period in the first century CE (either Camulodunum or Londinium, depending on what you technically call a city). If you look to Scandinavia, you are talking about another 900 years for the first cities. In the case for Britain, it took roughly 6000 years for 'cities' to travel about 4000km, and even then it was only because of external conquest. Sure Iron Age Britain had settlements, but large-scale cities were brought in by the Romans.

If Indigenous North Americans had had the additional 2000 years that Iron Age Britain had before the Roman conquest, they might have been able to develop cities. Regardless, being so far away from the epicentre of urban life, and in an increasingly marginal and hostile environment, really slows down the transmission of 'city' development.

[1] Joyce, R. & J. Henderson "Beginnings of Village Life in Eastern MesoAmerica" Latin American Antiquity 12.1 (2001): 5 - 24.

[2] Lohse, J. "Archaic Origins of the Lowland Maya" Latin American Antiquity 21.3 (2010): 312 - 352.

[3] Morin, E. "Early Late Woodland Social Interaction in the St. Lawrence River Valley" Archaeology of Eastern North America 29 (2001): 65 - 100.

As clarification, the "southern counterparts" such as the Maya you reference are, in fact, in North America. The Olmecs, Maya, Toltecs, Aztecs, Purepecha, Totonacs, Otomi, Zapotec, Mixtec, etc. were all involved in building complex, urban societies, and are all located in North America.

The distinction that separates Mexico and Central American countries from the United States is wholly a modern formulation which shows how cultural influences geography. Geographically, there is no reason to arbitrarily cut off North America at the Rio Grande, when the isthmus of Panama makes more sense. Culturally, however, it makes more sense to draw an arbitrary line between the difference between the Spanish (and Portuguese) speaking world and the English (and French) speaking world.

Americans are often taught that "their" history is one of colonists existing apart from indigenous hunter-gatherer groups. While this obviously ignores the extent of settled agriculture societies within what is no the boundaries of the United States, it also prevents a barrier to understanding the scope and diversity of indigenous cultures. If your frame of reference for "Native American" is small groups living in subsistence harmony with the land, it's difficult to integrate into that worldview some of the largest urban settlements of the pre-modern world which altered the landscape around them to fit the needs of their densely populated societies.

Also, the demarcation of the United States as "North America" belies how connected certain parts of what are now the United States and Mexico used to be, particularly the geographic oddity of having to travel northwest to get to the Southwest, the portion of the United States most affected by the cultures of Mesoamerica. In this region, we see evidence of Mesoamerican products such as chocolate and macaws in Chaco Canyon in what is now New Mexico, as well as other indications of Mesoamerican diffusion, such as ball courts, in the region. All of this is postulated to have been evidence of trade routes stretching along what is now West Mexico about 1000 years ago, connecting what is now the American Southwest to the what is now Central Mexico, both of which are in North America. Anthropologist Erik Wolf has even postulated that the Aztecs got their first taste of settled, urban society by serving as mercenaries for Toltec trade caravans headed north, a hypothesis which, at the very least, oversimplifies things, but still shows the interconnections between these subcontinental regions.

The diffusion of maize agriculture is another example of the connections between what is now Anglo- and Latino-america. After a slow, gradual development in Central Mesoamerica, maize exploded outward, first to the American Southwest and then out East. By the time of the BC to AD switch over, maize had started to creep into into the plains and forests of the Eastern Woodland area. Within a few centuries it was a staple crop, supplanting the earlier Eastern Agricultural Complex as it did so.

There's a bit of a popular fallacy that complex, stratified, sedentary society (the best definition of "civilization" I've ever come up with) requires a staple crop. This has really not been anthropological dogma for decades, which makes it amusing when pseudohistorians point to sites like Gobleki Tepe as completely unexplainable. In fact, anthropologists have long recognized the existence of "complex hunter-gatherers" who, by dint of the abundance of resources they could exploit, built up complex, sedentary societies. Ancient Mesopotamia is one such example, as is the Pacific Northwest much later on, whose salmon runs can be seen as a sort of quasi-replacement of the bounty and regularity of a staple grain crop. The Peruvian coast in the 3rd Millienium BCE is another example of a region without strong evidence of an abundant staple grain, which nonetheless built a complex society. The Norte Chico culture even built that hallmark of civilization, monumental architecture, in this case in the form of hug stepped pyramids. Even the early Olmecs have been argued as getting their start from an abundance of gathered resources, utilizing aquatic foodstuffs before refinement in maize agriculture propelled later Olmec groups to prominence.

While absence staple grain crop may not preclude a complex society, the presence of a productive, durable, and nutritious single crop (or suite of crops) certainly seems to make the development of large populations easier, with the implication of increasing sedentism, stratification, and specialization, all things typically associated with "civilization." There are many different strains of thought as to how such complex systems develop, though they often revolve around elite access and control of staple goods and/or the labor used to produce them. That the emergence of densely settled and stratified settlements in the Southwest and Midwest (e.g., Cahokia) post-dated the adoption of highly productive maize agriculture should not be a surprise.

By the time Europeans showed up on the Eastern Seaboard, they found maize-based agriculture groups up and down the coast and extending as far as they could fathom inland. What might have come from a growing population in the Eastern Woodlands undisturbed by such an arrival is a matter of speculation, but the fact of the matter is that high yield maize agriculture was still (relatively) new at the time of contact. I have no intention of straying into strict geographic determinism, but the more recent introduction of maize agriculture to what you are conceptualizing as "North America" can certainly be seen as factor for different population dynamics relative to what you are conceptualizing as "South America." Though, again, this conceptualization is a product of modern political and cultural social norms; Mexico and the Central American countries are geographically in North America.

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