Offhand, it's because it's a comfortable stance for a non-pistol shooter, it's almost like the finishing position of a punch with a pistol in your hand. In fact, if you watch these gangsters shoot, it almost like they jab the air with the gun.
Shooting stances are a funny thing, if you look at it through the ages, we go from a sideways dueling stace early on. From the start of the 20th century till the fifties, most favored a low squat stance with a one-handed grip. The transition to a two handed grip was only for long distance shots or if one had time to set up a shot. There was also more of emphasis on point-shooting, where you think of the gun as an extension of your wrist and hand/index finger and point it where you want the shot to go.
It was'nt till the 60's when the emphesis was placed on the "Weaver Stance" style of shooting, as it is more accurate, controls muzzle flip, and is a solider platform for follow up shots.