Why? They happen all the time, and have happened throughout history. There are plenty of geological fault lines in and around central and southern Europe. They haven't been all that active in recent years, but there are records of major and minor earthquakes in the region going back centuries. Obviously the fact that there have been many deaths from this quake is tragic for the people involved, but from a geological point of view this event was nothing out of the ordinary.
More for the number of dead in a pretty modern area than for the fact a quake happened.