They started this in the last generation where the founders cards had a different-but-similar connector.
In this generation, they would actually need 4x8pin to reach the 600W maximum these cards are supposed to be able to draw in a fully overclocked state, so really this just comes down to PCB space, but with the draw getting this high, the cable also has some signalling wires so the card knows how many Watts it should limit itself to.
As someone ignorant to this space, considering the PCB is already shorter than the heatsink, I assume the problem is more of re-arranging other components on the PCB than the actual physical space the final product takes up.
In this generation, they would actually need 4x8pin to reach the 600W maximum these cards are supposed to be able to draw in a fully overclocked state, so really this just comes down to PCB space, but with the draw getting this high, the cable also has some signalling wires so the card knows how many Watts it should limit itself to.