I think the reason it isn't a standard is multifactorial.
People have already alluded to the adjustment problem. Peter and I have a couple of student rigs that are set up to be easy to adjust, but it still takes time, and someone has to know how to do it. And a poorly adjusted rig isn't something where you can just swim up to the student and yank on a couple of straps and make it better -- it pretty much requires fixing out of the water, which can mean the student has an uncomfortable dive.
Backplate setups in cold water pretty much require a weight belt, and some students don't prosper with them. In addition, there is the significant difficulty of trying to don a heavy weight belt OVER a harness, so that it is ditchable. (If there is ever a time when I think somebody ought to have ditchable weight, it's when someone is a student in OW class, so that the instructor can render them permanently buoyant if necessary.)
In addition, for the student who is only going to be a warm water resort diver, the "traditional" BC is what he's going to rent, so it makes sense to expose him to it.
WHY the industry as a whole doesn't espouse primary donate, with either a bungied backup or even an Air2 type device, is what I don't understand.