what made yam specifically a man's crop?

Nearly every aspect of Igbo culture is gender specific. This even includes the food that they grow. Because Yams are a staple in the Igbo diet and helps the man support his family, it is given the higher status of a "man's crop".Check out this quote,

"His mother and sisters worked hard enough, but they grew women’s crops, like coco-yams, beans and cassava. Yam, the king of crops, was a man’s crop."