Because refrigerating superconductors to the cryogenic temperatures needed by current ones is expensive, severely limiting the applications they are used in.

• Metallic superconductors need cooling to the temperature of liquid helium.
• Copper oxide ceramic superconductors need cooling to the temperature of liquid nitrogen.
• Room temperature superconductors, if they exist, would need little or no cooling.

In a way, all currently existing superconductors are "low-temperature", but some more so than others. The traditional superconductors work up to about 20 K (or minus 253 Centigrade); more recent "high-temperature superconductors" work up to 100 K or so. 100 K is still minus 173 Centigrade, but it is much "hotter" than the traditional superconductors. The new "high-temperature" superconductors apparently work different than the old-fashioned ones; at least, the theory that explains the traditional superconductors fails to explain how the new superconductors work.