The Proteas were regarded to be among the favourites to lift the trophy in India, but they failed to advance beyond the group stages.
Asked why his side had struggled to live up to expectations, Du Plessis highlighted a number of wayward deliveries.
"We were poor with our extras. Every game, we had more wides or no-balls than opposition," he told reporters.
"That's basic and controllable. A World Cup is a do or die tournament and if you make mistakes, it's not like a series where you come back into the series."
In defeats to England and the West Indies, South Africa sent down 36 extras.