Consider patients who are unconscious (or awake but unreliable) due to head trauma, stroke, etc. You can't do a proper MR safety screening interview to find out if they have ferrous metal or other conductive objects in their head. The magnetic field can potentially torque an old aneurysm clip or piece of shrapnel, and the radio frequency excitation field can cause heating and burns if there are antenna-like conductors present.
MRI is slow and the images are easy to ruin if the subject moves around too much during the long acquisition time, so bouncing around in the back of an ambulance isn't the ideal setting. A 3D volume with whole head coverage and suitably fine spatial resolution can take several minutes to acquire, versus just a few seconds for X-ray CT. The MRI signal to noise ratio is also proportional to field strength, so low field systems like this are at a disadvantage for image quality. On the plus side, low field does mean less risk of moving ferrous material inside the subject.