Most probably slop of a trusses are considered by the external force on the roof , WIND ZONE, WIND FLOW., WIND PRESSURES AND FORCES ON BUILDINGS/STRUCTURES.
It differs based on o meteorological properties of the region. Hence, there are different criteria and regulations in codes. For instance, the wind load and snow load play a significant role in the slop of truss. In addition, the vertical earthquake component is an effective factor in the selection of roof.