[nickcw]

Cost.

The wiring for 48V can be a lot thinner than it is for 12V. As there is a square law involved for resistive heating it turns out that wiring for 48V can use 1/16th of the weight of copper as that for 12V.

A switchmode converter can be designed for 48V just as easily as 12V.

[op00to]

It's far cheaper and easier to just pluck a readily available 12V power supply off the shelf than it is to design one that will have limited applications outside of a single manufacturer.

[Arrath]

The inertia of established supply chains and industry norms can be a real mother to overcome.

Hobbyist computing could benefit from a move to 48V as well, if only to keep the problematic 12VHPWR from killing expensive video cards.

I say hobbyist because AIUI 48v is making inroads in server hardware but that's not my area.

[AlotOfReading]

48v automotive designs have been available from suppliers for a few years now.

You generally can't reuse non-automotive power supplies in automotive because the requirements are very different.