Why the ball not fall behind?
An object in motion remains in motion in a straight line unless acted upon by a force. Since you, the ball, the air in the car, all are moving at the same speed, you detect no unexpected motion. But if you throw the ball as the car goes around a curve, it will not fall back into your hand: it will fall toward the outside of the turn. Likewise, a helium balloon will move toward the inside of a turn, while you and everything else are thrown toward the outside of the turn. Try it and see.