Another idea is that in the 1700s,people in France and in the United States started to use wagons with several pairs of horses and to drivers to sit on the left and control the horse/ with a whip in the right hand, and it was safer if other vehicles passed on the left so the drivers could see the distance between vehicles. Some people say that in England, people always drove on the left, and in France, people always drove on the right.
In any case, Napoleon, a ruler of France, ordered people to drive on the right side in the early 1800s. France conquered many countries at that time, and so these countries drove on the right. But France didn't conquer England, so England kept its custom of driving on the left.