Early in the last century (think 1900 to the 1930s) when vacuum tubes and tube driven amplifiers, receivers, and transmitters were being developed, negative ground was standardized since it made much more sense to figure things out that way, from an electronics theory point of view, which itself carried froward from the late 1800s. (simplified: electrons moved, or jumped, from the negative 'ground' to the positive 'plate' in a vacuum tube)
Fast forward to the 1950s and 1960s, and transistor theory was coming into its day, and the engineers and designers stuck with what they knew: Negative ground.
Right around then, (and of course earlier) the automotive industry started putting radios in the dashboards, and of course, some cars were positive ground, which made things difficult when the radio was negative ground, and the vehicle was positive ground.
Of course there were some 'work-arounds': some radios were advertised as floating ground, some were 'kludged' up to be positive ground, and even Radio Shack sold a ground isolator for installing standardized negative-ground radios in (by then non-standard) positive ground vehicles. But during all of that time, the basic internals of the radios were NEVER positive ground, only the chassis connections were rewired to make it compatible. Electronic circuit design was always negative ground.
Eventually, the automobile manufacturers caught up, the light came on, and they said hey, lets just make these cars and trucks negative ground to be compatible with all this electronic stuff the customers want.
So really, the reason cars are negative ground today is primarily due to the early days of vacuum tube and electronic circuit development.
Just FYI.
