Mo':

From Country Life magazine:

"The Americans, of course, drive on the right. Initially, when it was a British colony, the inhabitants drove on the left, but following their rebellion in 1776, they forswore all practices they associated with their colonial masters. Of course, the influx of settlers from European countries who had been subjected to the dread influence of the French also helped." [The "dread influence" was that of Napoleon, who enshrined driving on the right in much of Europe).

So yanks drive on the right because their forebears were just peevish. Though it should be said that the change didn't happen all in one go.