Why do people slash and burn their crops

They didn’t slash and burn their crops. They cut down forests(=slash) and set fire to the rest(=burn). This freed up surface area for farming, and the ashes made the ground fertile. Trouble was, they didn’t use fertilizers or crop rotation. When they didn’t get good yields from their lands they’d simply slash and burn some more. Old farm land was left almost empty of nutrients, with high risk of erosion and poor chances for reforestation.