What made World War I much more deadly than previous wars?

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.