The hardness / softness of the material is not the relevant issue. It's the yield / ultimate tensile strength that matters. Also, the modulus of elasticity affects the flexibility of the bars. Copper tubing could be used, but it would have to be oversized and would be very heavy. Then, you would have to shim it somehow to fit available stem clamp sizes. Copper is a great material for sculptures, but pretty lousy for load-carrying structures.