Warwick Bray writes that the Mexicans understood the principle of the wheel but only used it for children's pull-along toys, not for transportation.
Warwick Bray, Everyday Life of the Aztecs (New York: Dorset Press, 1968), 87
Transports and communications
The Mexicans understood the principle of the wheel, but never applied it to anything bigger than children’s pull-along toys in the form of wheeled animals (25). The reason is probably not lack of inventiveness, but rather the absence of suitable draught animals and the ruggedness of the terrain.