Unfortunately, MRI can not be used for every patient. Those with pacemakers and ferromagnetic metal implants are not eligible. Also people with even mild claustrophobia can experience great discomfort during an MRI scan, which requires them to lie still inside a narrow tube. Increasingly many open MRI machines, which don't have tubes, are available. But the images they produce, though still superior to CT scans, are not as sharp.
That said, CT scans are still an integral part of every hospital system, particularly in emergency settings because they don't take nearly as long as MRI scans to produce. Also, they depict bone more clearly than MRI. So in cases of the unconscious patient with possible head trauma, CT is the way to go.