In a word, efficiency.
Starting in the era of the Model A Ford, cars has a 6 volt electrical system. That 6 volts was needed to run the lights, the ignition, the horn and the starter. That was it. Such accessories as the windshield wipers were powered by engine vacuum. The battery was charged by a small generator — generally putting out 20 or 30 amperes. Thus, a 30 ampere generator could put out up to 180 watts if the engine RPM was high enough.
Over time more and more electrical devices got added to cars. A radio, an electric fan for the heater, directional signals and more powerful lights. Higher powered accessories such as the compressor for an air conditioner was run off a belt from the engine as was a power steering pump. In 1955 General Motors switched to a 12 volt electrical system with Ford following in 1956. By 1960 the only major car manufacturer still using a 6 volt electrical system was Volkswagen. That actually made sense as there was no power steering and the heater fan was just “extra air” from the engine cooling fan. But, eventually, everyone was using 12 volt electrical systems.
Aside from growing electrical demand, once the majority of the manufacturers had moved up to 12 volts, most electrical devices for cars were available only in 12 volt versions. Such things as power inverters (to supply 120 VAC), radios, tape players and such. Thus, 12 volts became the standard for automobiles. As accessories got added higher output alternators replaced the old generators to meet the electrical needs.
As electric vehicles entered the market they needed higher power to propel the vehicle but they continued to use 12 volts for the accessories. Why? Because that was the standard. From lights to fan motors — all things that were needed in an EV — were readily available in 12 volt versions. But, there was a problem.
Accessories such as windshield wipers and power brakes had no source of vacuum so they had to be electrical. As the electric motor is not turning when your vehicle is stopped, air conditioners and power steering pumps had to be electrical. Add to that running the “computers” in modern EVs, entertainment systems and such and you needed a lot more power than was typically needed in an ICE car. But, the supply chain for automotive parts (not just for the consumer but for the manufacturers) was clearly locked into the 12 volt market.
In an EV there is no generator or alternator. There is a power inverter which runs off the main vehicle battery (typically somewhere between 120 and 1000 volts) and produces the 12 volts needed for accessories. This is not hard to do but at these higher currents larger wires are needed and voltage losses quickly add up.
It appears that Tesla is the first EV manufacturer to bite the bullet and go to 48 volts. That is, to go to suppliers of lights, fan motors and such and say “we want 48 volt units instead of 12 volt” for our new Cybertruck. At this point, Tesla has almost two million pre-orders for the Cybertruck which should be enough to inspire those suppliers to take the demand seriously. With lower wire diameters, the 48 volt products should eventually be cheaper and they will weigh less. It’s the right thing to happen. I expect that many if not most EV manufacturers will jump on board the transition.