Here in Maryland the natives had the "three sister" crops - Corn, beans, and squash. From what I can tell, corn's original plant originated in what is now Mexico, beans from somewhere in Central or maybe South America, and squash was also from somewhere around the same region.
From what I can tell those crops spread out both north and south, being essential crops to a number of cultures in the Americas. Potatoes, in spite of similar usefulness, did not ever seem to travel very far from Peru, however.
From my understanding trade up from Central/South America existed, as the Chief of the Powhatan evidently had a South American parrot as a pet when the English arrived in Virginia. One would think that over the centuries such a durable and useful crop would have "grown some legs", even before the Inka empire began its expansion.
Potatoes definitely grow at least up here - I've done that myself, even.
My best guess is that maybe potatoes don't grow as well as other crops further south and thus there was a dearth of land that could support the cultivars available at the time (thus there's no "land bridge" for them to creep along). This is a stretch to me.
Is there any formal speculation as to why potatoes seem to be so region bound compared to other staple crops?
While South America is well known to be the cradle of potato civilization, so to speak, there are potato species that are native to North America. In fact, there is evidence of at least one potato species being consumed in the American Southwest 10,000 years ago, but evidence is scant as far as whether or not it was domesticated/cultivated.
The ancestors of modern as-we-know them potatoes were domesticated in the Andes about 7,000 years ago. A spectacular variety were developed, partly due to the ingenious coast-to-mountain trading systems of pre-Colombian Andean communities. A number of different microclimates existed in these steep mountains (dependent on altitude, proximity to the coast, etc.), which allowed for a number of different landraces to be developed and cultivated.
As Europeans ransacked the Americas, they collected plant samples and brought them back to Europe in hopes to introduce them there (or to other wayward colonies). Naturally, the potatoes in the Andes would have been far better candidates than their North American counterparts; namely, the Europeans entered the Andes earlier than they did the American Southwest, the Andean potatoes were already domesticated, and the wide variety of cultivars would have a better chance of at least one taking to European soils to be further cultivated and developed.
As to why there was little (but not nil!) exchange between pre-Colombian North America and pre-Colombian South America, Central America is typically the answer. Warring and infighting amongst and between Central American societies would have created a dangerous barrier to much exchange happening, but the Derién Gap (the skinny bit connecting modern-day Panama and Colombia) was and remains a nearly impenetrable jungle. In fact, the Derién Gap is the reason the Pan American Highway still does not fully connect today - you have to ship your car (and yourself) in order to cross between the continents.
I am not a scholar, just a lowly potato nerd! I hope this begins to answer some of your questions.
Sources: https://www.pnas.org/doi/abs/10.1073/pnas.1705540114 https://unews.utah.edu/utah-home-to-earliest-use-of-wild-potato-in-north-america/ "1491" by Charles Mann
Potatoes evolved to grow in a tropical highland, where the climate is effectively temperate. The highlands are surrounded by lowlands that are decidedly tropical and potatoes don't do well under tropical conditions. This kept them pretty well contained until there was long distance trade to move them from the Andes to other temperate regions, although there was one major exchange of potatoes within South America. The Chilean domesticated potato, which is the major progenitor of the modern domesticated potato, descended from Andean potatoes that were brought to the Chilean coast and particularly the island of Chiloe, which is a temperate region. The Chilean potato hybridized with a wild potato, Solanum berthaultii, probably in Southern Bolivia, which introduced an important trait: the ability to form tubers when the day length is longer than 12 hours, which occurs during the growing season in temperate latitudes. The Andean potato mostly forms tubers only at day lengths of ~12 hours or less, which occur year round near the equator. So, even if you moved an Andean potato to a temperate climate, you would often lose the crop in a fall freeze before the plants had formed tubers. That is why the Chilean potato became the basis of modern potato breeding and the Andean potato remained mostly isolated in the Andes.