I don't know if this is the only reason, but superconuctors have a critical magnetic field that is also related to temperature (higher temp = lower magnetic fie

I don't know if this is the only reason, but superconuctors have a critical magnetic field that is also related to temperature (higher temp = lower magnetic field). So even if a material is superconducting at liquid nitrogen temperature, that doesn't mean it can produce a strong enough magnetic field for an MRI at that high a temp.