Because (like someone explained very well in another HW notice) you can't measure zero ohms. Any device to measure resistance ALWAYS would have a minimal value

Because (like someone explained very well in another HW notice) you can't measure zero ohms. Any device to measure resistance ALWAYS would have a minimal value that can measure. It's far more easy to detect superconductivity using the weird things that does all superconductors in presence of magnetic fields (ie, levitation).