Because the rest of the 6 series don't have an integrated plx pcie3 bridge that has the required spec?
I know internally the GPUs talk to each other via pcie3 but when they are pluged into a pcie slot should they not behave the same as any other pcie3 card like a 680 ?
I don't know the technical ins and outs of this but it is something that has puzzled me for a long time.