Why did World War I require total warfare?
It was a total war.
A total war is a war that is so large that every effort must go into it. This strategy is called total warfare, and is based on the principle of "giving everything you've got".
Because there were so many nationalistic, allied nations involved, it was a big and high risk war. In addition, trench warfare began, which was ridiculously elaborate and expensive.
This was a time period where factories were cranking out wartime goods instead of commercial goods (enamelware factories would stop producing pots and pans to make bullets) and people were rationing their food so they could send it to soldiers.