Agreed with the bayonet handle idea,and one other important factor,and I credit 2damoldforthis for pointing it out first.
With a muzzlestuffer,once you fire your shot,its time to reload.
Is good if you can still keep the action in front of you in your field of view.
Rifle butt on the ground,its good if the muzzle comes up near to the chin.
It is something a riflemaker considers...at least my mentor pointed it out to me.
One more thing.Think troops,combat,stress,and the nature of reloading a muzzle loader.A shorter rifle invites the face to be in front of the muzzle.
As this OP was about rifle design,also consider the change from a 60 yard or so rifle shot standing on your hind legs
And more what was seen in WW1.Still need a bayonet handle...but a longer range belly in the mud rifle.Pesky machine guns !
Then,the difference between ranks and long bayonets vs a mad brawl,individual combat,and close quarters.The forte to foible.Parry and get inside.The long bayonets were cut short.Rifles got shorter.