A coil spring is nothing more than a torsion bar that is wound up into a coil, the stresses in the metal are identical. Not all coil springs are progressive, only the ones that are wound with some of the coils closer together than the rest. As they come in contact it effectively changes the overall length. If you could easily shorten a torsion bar over its travel you would get a progressive rate too.
The only real advantage to torsion bars is with a independent front suspension that drives the vehicle, it allows for less height required to package the front suspension. The majority of 4x4 vehicles with independent front suspension used torsion bars at some point. Both my Dakotas have torsion bar front suspensions.
Why did Chrysler use torsion bars on cars? Likely just to be different, they did a lot of things to be different.
