according to the book of totally useless information:
The costom of driving on the left or right side of the road varies from country to country, and its origins predate the invention of the automobile. During the 1700's in England, Horse-drawn coaches were the main mode of transportation for urban dwellers. British coachmen used to sit on a seat on the right side of the carriage. if the coach traveled down the right side of the road, the coaches whip would have hit people walking along the sidewalk. by keeping to the left, the coachman had room to use his whip and the pedestrians were safe. the coach drivers could just as easily have sat and driven on the right, but that's not the way it turned out.
Cars in france, England's neighbor, drive on the right side of the road for reasons also related to the horse and buggy. Eighteenth-century French coachmen rode on the left side of the pair of pulling horses, and drove on the left side of the road. This caused the horses to pull to the left and up onto the sidewalk. They were subsequently ordered to stay to the right to protect pedestrians.
it makes practically no difference today which side of the road is driven on, as long as everyone uses the same side!
there you have it, the reason for the difference, it is just costom