Why is 12V Common for PSU

A short story

Long before solid-state electronics and vacuum tube electronics, lead acid batteries were and are the longest living battery technology. Being a multiple of 2V +/- 0.1 per cell or 2.36V on float charge or 14.2V, the 12V battery was not the 1st common voltage for cars. Initially it was 6V but as gas engines grew , they used 2x 6V batteries then became a single 12V pack for economical and performance value.

Meanwhile

Telecom used 48V (24 cells) to power ringing bells and powering millions of phones in local exchanges for landlines. They needed and still use 24 x 500 lb. 2V lead acid batteries designed for long life with ceramic and must all be balanced by monitoring specific gravity. This is the plain old telephone system (POTS).

Then Solid State Analog came along

Since lead acid batteries chemistry was chosen for best performance and operating at a float charge of 14.2V or more with tolerance, Automotive electronics rounded up to 15V for linear IC's requirements. So 15V and +/-15 for bipolar voltage is still used in datasheets specifications.

However for safe operating margins, 12V is still the most popular Analog voltage and is still used for wireless routers and all sorts of applications.

Rechargeable laptop voltages were all over the map for LiPo cell array voltages but eventually the industry settled on 19.2V as a "Universal" standard charger voltage which is converted for every different battery pack used inside.

Meanwhile for logic before and including TTL Logic IC's performed best 5V. But the datasheets will often specify then at the standard 10% tolerance of 4.5V or 5.5V and the same for 3.3V as 3.6V logic that works at 3V on Lithium cells.

Now personal computer CPU's operate below 1V for performance and efficiency advantages and only increase voltage on demand for power determined by the BIOS.

Who knows

While Lithium is most popular for portable applications, 12V lead acid in cars is still the most economical. My Mercedes SUV has 2 of them. One in the rear for auxillary use and the main under the rear passenger floorboard. There has been a lot of research to minimize copper costs and higher voltage car batteries have been considered but the cost of industry change must also be considered and the profits on 12V.

There is no guaranteed correct answer, but the "nominally 12V" available from 6 x lead acid cells and its adoption as the standard for most domestic motor vehicles is a powerful force in setting a standard in other areas. Some early cars used 6V, which proved far from ideal, and larger non-domestic vehicles use 24V (as Tony noted). But for many purposes "12V is king".

Regarding automotive use:

For a given power level 6V doubles the current and multiplies resistive loses by 4 for the same wiring. 24V reduces losses by a factor of 4 but for typical energy capacities increases cell count by 2 so the same size battery has more separators and manufacturing effort. For larger capacities (eg trucks) this may make sense.

At one stage the use of 52V car systems was proposed, but the madness faded, fortunately.

12V in lead acid batteries provides a balance between good energy storage, good power availability, and not too high currents at an energy storage capacity well enough suited for automotive use. It is very unusual to find a 12V automotive battery under about 24 Ah, 40 Ah is "reasonable" and 60 Ah on the upper end in most applications.