I'm not sure if this is the same as what you mention with 12V vs 5V, but Intel is pushing a 12V standard PSU design that is also used by the new Nvidia Founders Edition GPUs:
https://www.intel.com/content/dam/www/public/us/en/documents...
https://www.intel.com/content/dam/www/public/us/en/documents...
Lets say you make a design that has +/- 1V error. On the 5V circuit, that's 20% error. But on the +/-12V circuit, it is only 4.1% error. (Going much better than 5% or 1% error is nonsensical. Most resistors you'll buy are 5% error, and capacitors are maybe +/- 20% error). So you can see, +/-1V error is acceptable on +/-12V circuits, but maybe unacceptable on 5V circuits.
If precision is important, there are 1% or 0.1% resistors available, as well as matched-resistors (which have say 1% error, but all the resistors have the same degree of error, so your "proportions" remain consistent. You can manually-match resistors together with an accurate ohm-meter as well)
As such, +/- 12V is simply easier to make "precise" electronics on. Of course, the downside is that you've got leakage all over the place (more voltage means more power-draw)
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But now its 2021. Most parts, even extremely cheap "Jellybean" parts like the LM358, have low errors and low-bias. And USB's popularity as a 5V delivery mechanism has grown, everyone has a USB plug somewhere to use as the basis of electricity experiments.
So while electronic engineers way back played with +/-12V, today the assumption is that you play with 0V to 5V.
If you are already putting buck converters everywhere, it makes sense to raise the voltage coming from the PSU, because you can reduce the current supplied by the PSU. Only the power supply will be running at 12V, almost everything else will be running at a lower voltage. (Would you rather supply 120W at 12V/10A or 5V/24A?)