This is a really interesting question, but also one that is probably impossible to answer definitively. People lived in the region for more that 10,000 years, so any attempt to give an answer is bound to consist of generalizations at best. And from a historian’s perspective, it is really hard to answer why something did NOT happen (since there can be innumerable reasons for something to not happen), so perhaps a better question might be why did the societies that existed in the region take the forms that they took (as you allude to with your assute follow up questions about the environment and socioeconomic factors).

I’ll just mention a few things that I think are important. Before the Spanish arrived, the peoples of the Uruguay/Paraná river valley were either nomadic or seminomadic. Nomadic peoples from the Pampas and Chaco region raided, hunted, and gathered from the plains into the forests of the river valleys. Seminomadic groups stayed in the forests and along the rivers. They practiced slash and burn agriculture, raising a variety of crops, and moved from site to site when the soil at a particular place was exhausted. Often, they lived in groups of longhouses which could house up to 50 people each, though generally the individual bands didn’t get much larger than a few dozen people. Some indigenous groups also adapted to a predominantly fishing and gathering lifestyle, traveling up and down the rivers in canoes. Most of these groups supplemented their diets with other resources gathered or hunted from the surrounding environment. Raiding was common, and warfare was a prime concern for indigenous peoples. Population densities were low; there never was a massive population in the region (or at least there is not evidence of such development like we see in the Andes, what is today Northwestern Argentina, and probably along the Amazon).

Guaraní peoples about 700-500 years ago began moving south along the rivers out of what is today southwestern Brazil. We don’t actually know why they were migrating southward. Some have said that there was a lack of resources and overpopulation to the northeast; others have claimed that it might have been a religious search for “a land without evil” (although I personally think the evidence for this is thin at best), but regardless, they were certainly a dominant cultural and social force at the time the Spanish arrived. They grew maize and manioc among other crops in clearings in the forests and built small communities that could get as large as a few hundred people, which wasn’t all that different from other seminomadic groups with whom they mixed and/or pushed out of the region. The groups had organized, complex social structures and kinship ties. When the Spanish arrived, the first settlement of Buenos Aires had to be abandoned due to the ferocity of nomadic indigenous resistance (among many reasons), so the Spaniards moved north to settle among these more settled, agrarian Guaraní, a settlement pattern that resulted in the founding of Asunción as South America’s “mother city.” I don’t think the Spanish conquistadors would have been able to survive in the region at all if they had not found and received support from the more settled, less war oriented Guaraní.

Also important in my mind to your question is the process of urbanization and agrarian development that was gradually taking hold in the Southern Cone, but this was interrupted by the arrival of the Spanish. We do not have a way to know whether the lifeways, government structures, and crops from the Andes might have spread south and eastward (as they were doing slowly over time already). We don’t know if the Guaraní villages might have grown to become permanently settled cities that began to develop infrastructure or features of an organized state. Or maybe they would have continued with their seminomadic traditions. Or maybe something else would have happened entirely. We don’t know, and we don’t have a way of knowing, which is why historians try to stay away from “what if” history altogether.

Another reason that separates the Argentine Mesopotamia from the Old World Mesopotamia is the lack of domesticable animals. The Western Hemisphere as a whole is famous for having only a few native animals available for domestication. Neither the llama nor alpaca is particularly well suited to the local environment. Once the Spanish arrived, horses, cattle, and pigs spread incredibly rapidly through the area, but the indigenous peoples did not have livestock or beasts of burden before.

So why didn’t the develop large, urban, agrarian populations? I suppose the short answer is because they simply didn’t end up urbanized. The environment was suited for different lifeways. Indigenous peoples had to move now and then to find new soil for their crops. The area could support the small population, and intercine warfare kept indigenous peoples on the defensive.

Lastly, I feel like I should mention that an urban/agrarian society isn’t necessarily any better or worse than a seminomadic or nomadic society. For a long time, there was this teleological belief in Europe/US/Latin America that settled, agrarian states were the pinnacle of progress and the most advanced stage of human civilization (and all other lifeways were barbarian, inferior, and in need of European intervention). This is obviously a problematic view of humanity based on colonialist discourses that always privileged Europe above all others. But even without big cities, all of the indigenous societies of the region were complex, dynamic, and specially adapted for survival in their environment. Archaeological sites, remains of burial sites, and records left by the first conquistadors indicate that they were relatively well off. They had firmly held and complex religious beliefs. Early Spanish accounts describe the residents of the region as healthy, strong, and fearsome combatants. The had decentralized leadership -- not centralized states -- but they certainly had social networks, alliances, and kinship ties with many other communities/groups. Just because urban/agrarian societies didn’t appear like they did in a few other river valleys around the world doesn’t reflect negatively on the people who lived there or the societies that they built.

Further Reading:

The Cambridge HIstory of the Native Peoples of the Americas, Volume III: South America

La Argentina Aborigen by Raúl Mandrini