The confrontation between European settlers and Indigenous America, Hämäläinen writes in "Indigenous Continent", “was a four-centuries-long war,” in which “Indians won as often as not.”
He goes on to say...
"Spain had a momentous head start in the colonization of the Western Hemisphere, but North American Indians had brought Spanish expansion to a halt; in the late sixteenth century there were no significant Spanish settlements on the continent — only petty plunder regimes. North America was still essentially Indigenous. The contrast to the stunning Spanish successes in Middle and South America was striking: how could relatively small Native groups defy Spanish colonialism in the north when the formidable Aztec, Inca, and May Empires had fallen so easily? The answer was right in front of the Spanish — the decentralized, kinship-based, and egalitarian political regimes made poor targets for imperial entradas — but they kept missing it because the Indigenous nations were so different from Europe’s hierarchical societies. They also missed a fundamental fact about Indigenous warfare: fighting on their homelands, the Indians did not need to win battles and wars; they just needed not to lose them."
Are historians mostly on board with this analysis?
First of all, thanks to u/grip0matic for paging me.
Pekka's conception of Spanish warfare seems a bit on the scarce side, since he is not commenting on how he is basically describing skirmishes and guerrilla warfare, something completely within the Spanish military doctrine. In fact, it had been succesfully used in Gonzalo Fernández de Córdoba's campaigns in Italy, although the Great Captain did not call it guerrilla but "guerra guerreada". Why do I mention it? Because Hernán Cortés himself had been a veteran of the wars in Italy.
The most important factor on why the Hispanic Monarchy did not venture deep North is quite straightforward: lack of manpower.
The Iberian Peninsula, and more specifically the interior, has always been severely underpopulated. This is not only true for the 16th century, as one can clearly by the different censuses carried out throughout the period (censo de pecheros 1528, censo de millones, informaciones topográficas, censo de pecheros 1591, etc), but even for the very distant past as explained by professor Gonzalo Ruiz Zapatero discussing the topic of Celtiberian settlements. Furthermore, travellers of the 16th century like Andrea Navagero or Anzolo Trevisan took notice of how underpopulated Castile was.
Conquering the Aztec Empire and the Inca Empire was relatively easy, as Hernán Cortés and his uncle Francisco Pizarro exploited the weakness of mighty states that were on the brink of collapse within. The alliance with the nations that wanted the Aztecs and Incas to fall is what made everything possible, and why in recent times there is a strong historiographical shift towards the "native conquistadors" without whom nothing could have been accomplished. The Hispanic Monarchy also encouraged miscegenation, thus integrating the Spanish nobility into the existing aristocracies, solidifying the territorial control. Good examples can be seen in the Nobiliario de Conquistadores de Indias.
The lack of a sufficient population, either in Castile or the valley of Mexico after the conquest is what made the conquest of the territories in the current USA completely unfeasible. Lack of central power structure also played a part, as the integration into the local aristocracy would not have been practical, for the gains would have been minimal.
The Hispanic Monarchy then opted for the only possible or viable solution considering the available resources and manpower: the creation of a certain number of cities along the Camino Real, and the establishment of occasional alliances with some of the tribes up there in the North.
You'll be interested in this response from /u/bem-ti-vi on the ways the Spanish benefited from existing imperial Inca infrastructure