There are 2 main factors:
1. Science and Technologies
This is the time that artillery and rifles are being modernized. Bigger and tougher tanks, planes, bombs, etc. were developed to create a bigger destruction to infrastructures and enemies. Modern guns combined with highly destructive tactics ensured higher number of casualties was definitely a contributing factor.
2. Scale of the actual War
It involved more people fighting, bigger countries participated and the duration of the actual war.