Why do big companies like Volkswagen, Audi and BMW almost always decide
for linear springs?*
The only progression on their cars is in the bump stops.
I don't see any advantage on progressive spring designs though. Progressive
means softer on the first part of wheel travel and stiffer and the end of it, but
the much bigger factor of small bumps in the dampers behaviour. Better have
less slip-stick-effect and more advanced valve design. That's why a Bilstein B6
or even an Öhlins rides better for the given stiffness (compared to a similar stiff
cheap one).
Another reason is the springs needs to be controlled by the damper. With a pro-
gressive spring a damper would need to be progressive (on travel) as well. But
dampers aren't. They are very progressive to speed, but not to travel (I know
there has been the Monroe Sensatrac some time ago, but it hasn't been that
good as promised).
I'd guess there are more reasons, but I don't make the decisions at the above
companies and I strongly believe they know their stuff.
* The only Volkswagen I know that used progressive springs has been the Scirocco Mk.I on rear.