Mythusmage, I have to agree with Starman on the draft animals thing.
The ratio of force to feed you get out of a horse or ox is better than what you get out of a human. Add a horse collar or yoke and you get even more disparity.
The Aztecs did not have easy access to bison, and even if they had, bison are not the easiest animal to domesticate. To quote the article below, "Bison are very fast, powerful and mean...They are very hard to handle and they are very hard to own from an economic perspective."
http://www.law.virginia.edu/home2002/html/news/2002_fall/bison.htm
Keep in mind that it wasn't until the 1950s or '60s that ranchers effectively crossbred bison with domestic cattle to try and get the best qualities of both. The result was a 3/4 bison hybrid. Most people aren't aware that such crossbreeding took place.
As to the question of whether or not there was a pressing need to carry more than what a man can carry...the question is not one of need, but one of efficiency. The collection and distribution of grains and other agricultural products are more effective in bulk. An animal can actually pull far more than they would consume during the journey, and a single farmer can move a large volume of produce with limited assistance.