Because the morningstar is the only weapon I can think of that can allows a full strength build to oneshot anything outside of the heavy horses.
High pierce, decent speed (enough that you will never be outspammed), a little short (main balancing factor here), completely wrecks shields in no time at all, unbalanced so don't try a lot of feinting - all this adds up to a great weapon for a full strength build to go oneshotting, in my opinion. It also makes a pretty satisfying sound when you hit someone with it.