But it wasn’t Detroit. It was, to quote you, “all of them.”
Airbags were optional equipment, available for a price. Even standard on higher-end cars. The fight was against the legislation requiring them on passenger vehicles and light trucks.
Sure, there’s a profit motive there, but at the time, people weren’t even wearing seatbelts, which arguably would have save more lives.
It was because that was the basic, mainstream Chevrolet from back then. In '59 the Biscayne, Bel Air, and Impala were basically trim levels of the same car.
As for it being “slightly unfair” that the Bel Air had an X-frame, that doesn’t matter. 1) that’s what was on the road back then, and 2) it doesn’t really matter what kind of frame in under there when a collision just rips the body free of the frame mounts and collapses. The whole point to modern automotive structural design is to design the forward structure to crumple in a controlled fashion. The front “subframe” rails are designed to collapse like an accordion; the energy from the collision is partially dissipated as the metal being accordioned gets very hot. It also reduces the mechanical shock impulse felt by the occupants of the car, which reduces the chance of neck injuries once the colliding vehicle reaches the rigid passenger cage.
Put it this way- there are four times the number of cars/trucks on the road today vs. 1959, yet our on road fatality rate is less than half. That video shows why. Complaining about frame types is completely missing the point.
If you prefer graphs to videos
The video was created to tell the same story more dramatically.