Granted, agricultural societies existed in North America too, but aside from the pueblos in New Mexico and some urbanized communities associated with the Mississippi Valley Civilization and its offshoots, the population density and urbanization didn't compare to Mesoamerica and the Andes. This has a lot to do with why present-day Mexico, Peru, Bolivia, etc. have large indigenous communities relative to Canada and the US: there were higher populations there to begin with. It doesn't necessarily mean that the proportion lost to disease and warfare was any less.
To bring this back to the parent comment: The anthropology museum [1] in D.F is incredible both in depth and scale. It covers the history of the city and area so thoroughly. I had no idea about the extent to which civilization had persisted in Mesoamerica before I spent half a day wandering around this museum. It's such a fascinating history and it's available to see right there through the recovered artifacts and the current ruin sites both in D.F and just outside (such as Teotihuacan [2]).
Oh, and if you go stay here: http://theredtreehouse.com
[1] http://en.wikipedia.org/wiki/Museo_Nacional_de_Antropolog%C3... [2] http://en.wikipedia.org/wiki/Teotihuacan
Cortés arrived in Mexico with a few hundred men and allied with locals to overturn the Aztec empire, but natives were always the majority of his alliance. And natives have always been the majority of the ancestors of Mexicans, even today. The pre-Hispanic civilization persists in every aspect of life from street grids to artistic styles. There are still more Mexicans living on canals than Venetians.
I was bicycling through the mountains of Milpa Alta one day; Milpa Alta is inside the Federal District of central Mexico City but more rural by virtue of steep mountainsides rising straight up from dense valley development. I came to the town of Milpa Santa Anna where I rode to the charming central square. Narrow old streets are colorful and lively there with some cross streets passing each other on bridges due to the steep hillsides. There was a public notice board in the central plaza where all notices were posted -- ten miles from the denser parts of Mexico City -- in Nahuatl -- the original Aztec tongue -- as well as Spanish. Except there was one notice posted only in Nahuatl. I can't tell you what that one said because I don't speak the language, but it really makes you feel like a foreign traveller in the deep past when public notices are still written in Aztec.
It's not just large relative to Canada and US, it's absolutely dominant. In Mexico, 88% of population is Native or mixed-Native. Compared to less than 2% in the US. http://en.wikipedia.org/wiki/Indigenous_peoples_of_the_Ameri...