According to Richard Bulliet's History of the World lectures (available online), four-wheeled wagons are globally less common than wheelbarrows or two-wheeled c

According to Richard Bulliet's History of the World lectures (available online), four-wheeled wagons are globally less common than wheelbarrows or two-wheeled carts because: (a) extra wheels add considerable weight and friction (b) wagons have a larger turn radius (See also https://en.wikipedia.org/wiki/Wagon_wheel_(transportation)#Terminology_and_design https://en.wikipedia.org/wiki/Wagon_wheel_(transportation)#T...) TFA answers the question "why a wheelbarrow and not a two-wheeled cart": "Compared to a two-wheeled cart or a four-wheeled wagon, a wheelbarrow was much cheaper to build because wheel construction was a labour-intensive job. Although the wheelbarrow required a road, a very narrow path (about as wide as the wheel) sufficed, and it could be bumpy."