One obvious reason is that using a staff is meant to be different than a wand.
Another one is that staves, being able to serve a dual purpose as a catalyst for Warfare skills, should have some downside relative to wands(such as a limited ranged potential)
It is also worth considering that while Wands were from the start meant as primary offensive weapons, Staves are generally meant to only fill out the space within a turn where you cannot cast a spell.
