Mortal Wounds and Dev wounds

Devastating wound and mortal wounds are basically the same thing only dev doesn't spill over... why didn't rules writers just say that mortals don't spill? What functionally would change? Single damage would still work and multi damage would work as Dev wounds do now...

Instead we have this weird two tier system. Just wondered if I missed something as to why they added a new ignore save wound when they didn't have to?