Another way to put it: Cooper pairs become _bound_ objects. And so their energy levels are quantized, and to excite them, you need to apply more energy than is

Another way to put it: Cooper pairs become _bound_ objects. And so their energy levels are quantized, and to excite them, you need to apply more energy than is available from the thermal motion of atoms in a superconductor. This doesn't happen with unpaired free electrons because their energy spectrum is pretty close to continuous.