Gas
Airplanes
Submarines
Zepplins
Tanks
Trench warfare
Flamethrowers
Did you know what the Red Barron was when the war started? He was a lancer. As in a guy on a horse with a lance.
Imagine being a Tommy at the Somme. You go over the top and start walking. You can’t see the enemy trench, the grass is too tall. Then your buddy goes down. Then you go down, and from a machine gunner who can’t even see you.
The generals didnt know how to fight in tandem with their new technology. That and “Plan 17” was just so awful.
Many, many reasons. Most horrific war ever fought. Napoleonic tactics with modern weapons.