Tough touched on some good points, especially his mention of disease and population. i would like to add a little bit more concerning the native population in North America.
the population density in North America was
incredibly low, especially when compared to that of Central and South America. with their hunting/gathering lifestyle, it took
tens of square miles to support a single American Indian in what is now the USA (and this is a point i'll touch on again, later). furthermore, until Indians in the far south saw the Spanish riding horses and stole some for themselves, there were absolutely ZERO Indians living between the Appalachian Mountains in the east, the Rocky Mountains in the west, the Rio Grande river in the south, and north to the Great Lakes.
why? without horses, the distance between reliable water sources was simply too great for humans to travel on foot. all that area (the Great Plains, and so forth) was only inhabited AFTER tribes such as the Sioux, Kiowa, Cheyenne, and Shawnee got access to, and learned the value of, the horse. that means
no one lived there until after white men came to the New World.
another point of interest is the difference in the efficiency of the two lifestyles. Indians required tremendously large areas of land to support a small population. this also required a lot of travel and placed a heavy emphasis on unreliable food sources. conversely, a white farmer could grow enough food on a relative handful of acres to support a large family. this didn't require travel and was considerably more reliable. furthermore, this large abundance of food created by a single source allowed specialization of skills in the white population (and consequently more advanced technology) that was non-existent in the native population.
waterbed, i hope this helps.