Car bumpers

I would guess the EPDM helps it recover from minor impacts while the PP adds some rigidity. I would assume the blend is chosen to provide the properties required in a bumper. So think of the properties you would require in the ideal bumper and work from there.
Cost, compatiblity of materials, weight and other factors will come into it too.

Del

along with Del, I suspect the formulation was derived from early plastic bumper protests of sun fading and embrittlement and impact susceptibility.

Polyurethane microcellular foam was the standard for many years, but the foam was more difficult to paint and the PU was much more expensive that the current PP + EPDM rubber mix. The current rubber forms can also be recycled, depending what the diene monomer is in the EPDM.

why not people make bumpers have same material, same shape, same size, same distance from floor?