why does the headlines usually use the word magnitude rather than intensity of an earthquake?
The Mercalli Scale is based on observable earthquake damage. From a scientific standpoint, the magnitude scale is based on seismic records while the Mercalli is based on observable data which can be subjective. Thus, the magnitude scale is considered scientifically more objective and therefore more accurate.
An intensity scale measures that physical damage done by an earthquake. A magnitude scale measures the energy that the seismic waves send out.