Composite Leaf Springs

I dont know much about composite leaf springs but everything I read sounds like this would be the way to go. No sag, no corrosion, longer life due greater resistance to stress, 70% weight reduction, less friction/noise, smoother ride... so why do we not all run composite leaf springs in the rear? Why is this not the big ticket item for custom leaf packs for the rear? What am I missing?

 

From what I read they are more linear and less progressive. They flex fine and are standard on some GM cars, trucks, vans... in a low end form. Makers claim you get a linear coil spring type ride and it eliminates all the negatives with leaf springs. Soooo... I am wondering why this is not a HUGE market for desert guys. I understand why its not so great for load carrying but if that was not anted the Alcan and Deaver non overload leaves would not be as popular as they are.

 

They are using these on some pretty high end high performance rides now. Off the top of my head I know the new CTS-V uses a single leaf composite for rear suspension. Corevettes also have been using them for a while now.