Here's a lump of 100% Cu and another lump of 50% Cu and 50% "other".

If purity doesn't matter, they must have the same properties? Why not just use 0.0001% copper in all our applications and save millions on material costs.

One of the theories for the behavior of this material explains that the copper atoms preferentially form a structure that does not superconduct. The structure that does superconduct is tricky to achieve due to the energy levels. (Paraphrased summary)

It's likely that a poorly prepared sample will have discontinuous regions of both superconducting and non-superconducting material. If that is the case, you won't observe superconduction.

It may be that the non-superconducting material does superconduct at low temperatures, which would mask the purity problem.

The truth is that quite literally nobody fully understands what's actually happening here. That's kind of the point of all this experimentation