Why are the biggest earthquakes not the biggest killers?

No it isn't. There are several factors which contribute to earthquake fatalities: the size of the earthquake, the amount of population in the affected area, and the quality of construction of the buildings in that area. So, a huge earthquake taking place in an unpopulated region of Alaska, for example, would cause few if any fatalities, whereas a much small earthquake in Haiti caused lots of fatalities, due both to density of population and to poor construction standards.