One reason for the low death rate was the Japanese police, in which every officer is a paramedic. This probably saved many lives. But the biggest reason for the low death rate is the incompetence or modest aims of the terrorists. The poison was found lying in puddles on the floors of subway cars. Despite the name nerve gas, these things kill when tiny droplets land on your skin.
Mere vapor gives a small dose.
It could easily have been much worse, and the means to make it much worse are well known and available to all. We cannot count on the good will of terrorists. The same well-known and cheap techniques which could have turned the Japanese subway into a mass grave could be used to spread various infectious diseases. They are called filoviruses, and the diseases they cause are called hemorrhagic fevers. For vivid descriptions of these diseases, buy The Hot Zone, by Richard Preston.