[rotorwrench]

6-volt systems always needed all the help they could get for reliable service. It was known after the Model T era that the flow of electrons worked better for positive ground. Once the 12-volt era change came in, it was determined that the higher voltage would be fine using negative ground. A lot of electronics were already based this way for non automotive use so it was natural to go this route. In the UK, they still used positive earth on 12-volt systems for a while since they went to 12-volt first but it was eventually changed over for essentially the same reasons.