Is there an explanation as to why there were Empires such as the Inca's and Aztec's in the Southern Hemisphere, and yet no such Empires in the North?

First time posting here, but I think I can give it a go! Hope this helps. If you don't want to read it all, there's a very short summary at the bottom. Edited for links

On a technical note -- the Aztec empire was solidly within the Northern Hemisphere (as was part of the Inca Empire), and the Aztec, Toltec and other Mesoamerican states were also on the North American continent. But it seems like your question meant more to address the difference in empires between what is now Latin America and the US/Canada/Northern Mexico, so I'll talk more about that, and refer to that area as North America for the sake of convenience.

Part of the answer to your question is that our records of North American peoples and civilization are much spottier, for a variety of reasons. First, Andean and Mesoamerican states were encountered by Europeans earlier than North American ones were. The Inca and Aztec were already conquered by the time major expeditions into the modern-day U.S. began, and Canada was even after that. Coupled with the fact that there isn't any writing from indigenous North American peoples, we simply don't have as many records about that region. So there might have been more empire-style states in North America than we realize.

Europeans arriving in the Gulf of Mexico first also meant that the infectious diseases which decimated the Americas' indigenous populations spread outwards from that place. Importantly, those diseases spread faster than European exploration. By the time Europeans arrived in many locations -- before they could write records or witness native states -- disease had already ravaged the region. One of the more dramatic examples of this is in fact the Inca empire. When Pizarro arrived, hundreds of thousands (at the very least) of Inca subjects had died before any European had been to the empire. Perhaps most importantly, one of the Inca smallpox victims was the emperor Wayna Capac, whose death led the the Inca Civil War between his sons Waskar and Atawallpa, which tore the empire apart and facilitated the Spanish Conquest. The regular lives and societies of the indigenous Americas were largely disrupted before Europeans arrived to record information that we might see now

Second, we have to recognize the malleability of the word "empire." Do we mean a centralized government that actively restructured and consolidated conquered areas in its own image, applying government power across the state? If so, the Aztec "Empire" doesn't really count -- their state was much more one of forced tributary relations, and didn't necessarily restructure conquered polities. The Inca Empire might be the most comparable to what an Old World tradition might call an "Empire," but we have to be careful about how we use the word. Cultures develop different tactics of domination, and "empire" might not be the right one for the societies that developed in much of North America.

Having said that, there is increasing evidence that there were much more complex states in North America than archaeologists and historians have historically realized. Perhaps most famously, discoveries in Cahokia and other urban sites from the Mississippi area are rewriting the archaeological record. Here you had complex urban societies which waged war and had centralized governments and religions -- maybe, if we had more information, we could call some of these states empires. It seems that there were at the very least governments and individuals in "paramount chiefdoms" who commanded power and significance over hundreds of miles. Missisippian societies built massive earthworks, including pyramids. Along with archaeological findings, the accounts of Spanish conquistador Hernando de Soto provide insight into that area. His expeditions explored it before those states were completely wiped out by disease, and are simultaneously educational and thrilling, with stories of the Spanish force moving through alternatingly hostile and friendly towns and cities that they can't comprehend, and fighting against thousands of Amerindian soldiers. There were also complex and concentrated polities in the American Southwest. Ruins such as the 500+ miles of Hohokam irrigation canals, Chaco Culture National Historical Park, and Mesa Verde all point to this.

But I'm getting long-winded, and I'll try to directly answer your second-to-last sentence. It does seem to be generally true that, the farther north you go, the fewer complex states and certainly fewer empires there are in North America. A large part of this is simply environmental. It is extremely hard to grow crops in much of Canada and the northern U.S., making it difficult for a possible empire to grow. Population densities are lower and people are generally more dependent upon hunting and gathering, because it's difficult to do anything else. Exceptions to this rule include the Pacific Northwest, where powerful chiefdoms developed, but this area is warmer than other North America locations at the same latitude and also supports an unusually high amount of food from sea and land resources that were exploited. This is also the same reason that extreme southern South American peoples were also generally hunter-gatherers. A more historical reason for the phenomenon you're wondering about is the location of the New World's "cradles of civilization." Urban societies were original inventions in the New World along the Peruvian Coast, southern Mexico, and possibly the Amazon Rainforest (according to extremely new theories). Cities, and states that could become we might call "empires," spread outwards from those locations. So cities and urban states were still spreading northwards from Mexico when Europeans arrived. We see proof of this slow spread, and how much it could change societies, in things like the presence of maize among the Iroquois: traveling north from Mexico, maize reached the Iroquois in the 12th century, precipitating an agricultural shift and huge societal change that allowed for the creation of the Haudenosaunee, or Iroquois Confederation. So the factors that allow for urban civilization, and by extension empires, were less complete and newer the farther north you go.

To wrap it all up: Our records are weaker the farther north you go, because those areas were encountered by Europeans later (and were generally encountered after disease devastated them), so there might have been complex states we don't know about. Additionally, it's a myth that there weren't complex states north of the southern U.S. border, as evidenced by the Mississippian states and Southwestern U.S. But, holding in mind that the term "empire" the way we think of it simply may not be the way North American states ended up operating it, it is true that imperial-style states are less common the farther north you go. This was largely due to the lack of resources, and even more importantly the fact that the social inventions and factors leading to possible imperial states spread outwards from Mesoamerica, creating a kind of south to north gradation of potential-empire societies.