They probably still can't carry the necessary current without helium cooling. There are some NMR spectrometers (which work essentially by the same principles as

They probably still can't carry the necessary current without helium cooling. There are some NMR spectrometers (which work essentially by the same principles as MRI machines) with high-temperature superconductors, but those are still cooled by helium. The lower the temperature is the more current you can pack into those. Could also be that it doesn't make economic sense yet, they're still pretty new compared to classical superconductors.