What were the main reasons for the failure of Mao's Great Leap Forward?

Irrigation. Substantial effort was expended during the Great Leap Forward on a large-scale, but too often in the form of poorly planned capital construction projects, such as irrigation works built without input from trained engineers. Mao was well aware of the human cost of these water conservancy campaigns.

The Chinese were mainly focused on manufacturing and as a result destroyed many acres of farmland to build factories. As a result a famine swept China. Many cultural artifacts such as temples and mosques were destroyed by CCP soldiers.