Because they have very little mass (and therefore less momentum) compared to their size, so that their inertia is greater than that of either you or the car. Damn, I've been trying for several minutes to remember the equation (it's not E = 1/2 m v(squared), but is related).
Helium balloons can predict the future
In my opinion, the easiest way to understand this is to think of the air in the car as water. When the car accelerates, the water all rushes towards the back of the car and pushes the balloon towards the front.
It is 'displaced' by the air that's moved to the back of the car when it accelerated.