Why were they in technological stasis?
Jared Diamond's "Guns, Germs and Steel" covers this. But here are a few points that he made:
Relative isolation: People often adopt ideas and innovations from neighbors. In many cases an innovation is only developed in one area, but then spread widely and is adopted by many other people. This means groups of people with less contact with other groups tend to have a technological disadvantage because they miss out on the chance to copy the innovations developed by other societies. With that said, ideas move more easily on an east-west axis than a north-south axis. If you move east to west or west to east around the world, the climate tends to be relatively similar, although factors like ocean currents and altitude cause climatic variations. So the climate of the northeastern United States is relatively similar to that of northern China, which is on the other side of the world. But if you flew south from New York to the Caribbean, the climate is completely different, even though the Caribbean is much closer to New York than China is. As a result, people tend to prefer to migrate and trade east to west or west to east, to places where the climate is familiar, rather than moving north to south or south to north, to places with unfamiliar climates. Since ideas and technology tend to spread with people, that means ideas tend to spread from east to west or west to east rather than north to south. In Eurasia innovations like gunpowder spread all the way from China to Western Europe through intermediaries (east-west axis). But the Americas has a north-south orientation (look at a map). So innovations didn't spread as effectively from one part of the Americas to the other as they did in Eurasia. In Eurasia, for the example, the 2 main areas where writing systems were developed were the Middle East and China. Middle Eastern writing systems were copied and adapted by Europeans (Greeks, Romans, etc.), Ethiopians, Nubians, and possibly by Indians. Northern Indian writing systems were copied in southern India and the southern Indian writing systems were copied in Southeast Asia (i.e. Cambodian and Thai). China's neighbors, like Korea and Japan, copied Chinese characters. So most people didn't invent writing in Eurasia, they copied and adapted pre-existing writing systems. But in the Americas, the north-south axis meant ideas didn't spread effectively. So the Mayan hieroglyphics weren't adopted and adapted by the Iroquoian and Algonquian people of the northeastern USA, or by the Incas in South America. So technologies developed in one part of the Americas, such as Mesoamerica or the Andes, tended to remain confined to that region instead of spreading across the Americas.
Less advanced food production: Native North Americans' food productive system weren't as advanced of those of Europe. This meant they couldn't support populations that were as big and specialized as those of Europe. Why was this? It was because the Americas were isolated by other parts of the world, so that domestic animals and plants didn't spread from other hemispheres. In Eurasia, the Pacific and Africa, plants and animals were domesticated in one area and then spread across the Old World- chickens from Southeast Asia spread to the Middle East, Europe and Africa, for example. Eurasia had lots of different native animals and plants suited to domestication. The Americas, on the other hand, had very few animals suitable for domestication, and a limited repertoire of crops. The main crop of North American Native farmers was corn, but they didn't have many other crops. Potatoes, grown by Natives in South America, never spread to North America because of the north-south climate difference barrier. With few crops and almost no domestic animals other than dogs, Native North Americans just couldn't support the large populations and high levels of specialization that helped to make Europe so advanced.