[SteamWolf]

Napoleon.
Essentially.

From what I recall when this has come up, most of the early world under British rule would pass on the left when pulling wagons. Napoleon's hatred of anything English saw him decree that the French would do the opposite. Then when the French expanded colonisation this continued. Generally anywhere with French heritage drives on the right, British on the left.

Considering the USA was originally Fench, English and Spanish colonies depending on which area, it was always going to be a close thing as to which side they would end up driving on. As it turns out the right hand (wrong :P ) side due to large teamser wagons and the way they managed them (according to wiki).