48V is kinda industry standard, and just below "low voltage" thresholds is most jurisdictions (often 50 or 60V). The cards have switching stages anyway, so it's not that much change (but probably means different power transistors)

There's not exactly a right choice of voltage per say, it's more of which tradeoffs you are willing to accept. Nothing is free, and higher voltage means more noisy, complex, and expensive SMPS power supplies.

Solving for the problems of higher current, lower voltage DC we use in ATX format supplies today requires higher gauge copper, but the cable run between the GPU and PSU is so shot that the cost of extra copper could very well be less than the increased cost of a higher voltage supply.