The issue isn't really impurities and noise, quantum error-correction solves that problem. The issue is that the supporting technologies don't scale well. Superconducting qubit computers like google's have a bunch of fancy wires coming out of the top, basically one for each qubit. You can't have a million wires that size, or even a smaller size, so the RF circuitry that sends signals down those wires needs to be miniaturized and designed to operate at near 0K so it can live inside the dilution refrigerator, which is not easy.

Microsoft's technology is pretty far behind as far as capacity but the scaling limitations are less significant and the error-correction overhead is either eliminated or smaller.