I have wondered this myself...I think (and I am not 100% sure about this...) is the fact that it doesn't have a rotating back like the Mamiya RB67/RZ67, so using a wlf is a bit hard ,having to turning it on it's side and all. This can be "solved" by using a prism finder....but a lot (if not most) prefer using a wlf.

There might be other reasons...