I wonder why they didn't do what the scandinavians did with reindeer, and use deer for draft animals?
Probably had almost everything to do w/ the mountainous terrain where the Incas lived (Andes). Pack animals are just more suitable for that sort of terrain. Annual aver. rainfall at Machu Picchu (lost city of Incas) is approx. 85 inches. Translation--lots of road & bridge maintainence; narrower roads & bridges require less effort to build & maintain. Also, a good deal of the farming was on narrow mountain terraces, not the ideal situ for draft animals & plows. Even today, the Sherpas rely extensively on the yak as a pack animal in the Himalayas.
The other thing is that when you live in the mountains, if you invent the wheel, you also gotta' invent brakes. Coulda' been a case where all the Incan mechanical genuises kept getting killed in cart crashes...