1) Cinematographic reasons: It made an impressive scene even though it's highly unlikely that someone is able to get out of the car with even one machine gun bullet inside his chest.
It was FFC's homage to Arthur Penn, the director of "Bonnie and Clyde," whose famous "dance of death" scene at the end of the film became an icon of violent cinema. The Thompson submachine guns use by Sonny's assassins fired .45 ACP rounds. This ammo is designed for maximum knockdown power. Even one hit would have completely disabled Sonny.