Year 10 Physical Science: Vehicle Safety: Side Impact Protection

In a frontal crash the car has substantial amount of crumple zone distance but in a side impact crash there is minimal distance to absorb the crash energy.

Britannica entry explaining the physics of absorption.

Britannica entry explaining the physics of collision and energy absorption.

Occupants are particularly vulnerable in a side crash, since the bodywork at this point offers very little energy absorption by means of crumple zones. Thus the sides must be designed with maximum stability, so that the passenger cell is deformed as little as possible.