Why negative earth and not positive earth on a vehicle?

Standardization

The electrical term "earth" or "ground" is a connection that runs through the entire electrical system. It may or may not be connected literally to the soil. When a system is "negative earth", it means the electrical earth has a negative polarity. Power can then be run back and forth using only one wire carrying the positive polarity. Similarly, electrical signals need only one wire. At the destination, the circuit is completed by connecting the "negative" terminal to earth or ground.

Until the 70s, cars were built with either a positive or negative earth, with popular British vehicles being among the last to be built with positive earth. Negative earth was adopted globally by automobile manufacturers for the sake of standardization. It was the introduction of radios in automobiles that may have seen negative earth automobiles being favoured. It was also thought positive earth vehicles rusted quicker.

The US telephone system also uses a positive ground, after starting out with negative ground. However, the trolley cars in Atlanta, Georgia were causing electrical induction interference, which led to the phone cables literally falling apart. Every kind of solution was tried, but they all failed, until as a last resort the phone company flipped the polarity, and it has stayed this way till the present day.