Your speed is better. You hit a man under the chin with a speed. You take his head right off. And if you're stuck in a man's ribs, you have to kick him to pull it out. There are multiple stories of the battlefield where two guys drive their bayonets into one another at the same time, right? And they both die. Think about your entrenches. There's not a lot of room, right? You're in close quarter combat. You have a long rifle with a bayonet. That means you need room to attack and advance and kill someone with a bayonet. Hard to do in a trench when somebody's jumping over the trench to kill you. And a shovel is much more effective.