Because the child that Draupadi bore to Bhimsena was born after Bhima had performed one thousand Soma sacrifices, he should be called the great bow-man Sutasoma.
सुते सोमसहस्रे तु सोमार्कसमतेजसम्। सुतसोमं महेष्वासं सुषुवे भीमसेनतः॥