Why their sudden inward turn, and why the Chinese state’s abandonment of the oceans? Some historians, like Bruce Swanson, cite a power struggle within the bureaucracy between eunuchs and conservative neo-Confucians, dubbed “continentalists,” with the eunuchs ultimately losing out. Another important factor was probably China’s reopening of its enlarged and completely renovated Grand Canal, an extraordinary feat of engineering that connected northern China to the increasingly populous breadbasket of the south. At eleven hundred miles long, the canal was controlled by numerous locks, much like New York’s Erie Canal, which measures only one-third its length and was not built until four hundred years later. In 1415 the state banned the shipment of grain to the north by sea to compel use of the canal, for which thousands of barges were built. Such a decision would have dramatically reduced the need for shipping, and hence shipbuilding and the maintenance of fleets, leading to the halting of oceangoing ship construction altogether by Yongle’s successor in 1436.
That is from the new and interesting Everything Under the Heavens: How the Past Helps Shape China’s Push for Global Power, by Howard W. French. I believe the definitive economic history of the Grand Canal remains to be written, it will be a major achievement when it happens.