A 2 way speaker has a bigger physical demand put upon the 2 drivers of a tweeter/mid bass. Think about this...those 2 drivers have to make ALL the sounds!
The load is reduced when you divide the frequency and add a speaker to handle just the mid-range and a speaker for just the deep sub bass. The mid-base driver isn't jumping around as much trying to reproduce everything when the load has been greatly reduced by adding a dedicated mid-range and a sub-bass driver.
The same reason why a subwoofer is designed to handle ONLY the deep bass. With the addition of a high pass filter the mid-bass driver can breath easier and has much less movement to the cone surface. Less distortion is the goal.
4 way speaker... is a tweeter/midrange/midbass then add a subwoofer.