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.

The bigger reason is probably to save board space as nvidia have been shrinking the form factor of the actual boards so they can dedicate more room to flow-through cooling. Connectors take up a lot of board edge.

The new spec does also have data pins so the devices can negotiate for power which could in theory reduce issues and inform the consumer better about limits with future PSUs and devices.

So there are advantages to the new spec, both specifically with the form factor and more generally.