The reasons include the driver's eye movement required for visual contact is less and it is easier to drive on a narrow road because the body width is narrower than that with side mirrors on the doors.
理由として、視認時の視線移動が少量で済む事、ドアミラーと比較して車幅が狭くなるため、狭い路地に出入りし易いなどが挙げられる。