It all goes back to the good old days of the horse and carriage. As we are probably all a tad unfamiliar with this form of transport it is not unreasonable to have forgotten that any coach driver of old would have driven his carriage through the streets of Britain with a whip held firmly in his right hand. The more horses in front of him, the longer the whip he would need to carry. Excellent as it is for controlling horses a whip is a less desirable implement when pedestrians are in close attendance.
So it was that on roads which were wide enough and where pedestrians were in evidence, the practice was to keep the carriage over to the left so as to keep the whip towards the centre of the road and away from unsuspecting street urchins. Of course when carriages came to pass each other, they naturally passed each other on the left, keeping the whips central. So as the motor car came into being, drivers of these vehicles maintained these rules, keeping to the left to avoid other carriages - horseless or not.
So maybe the question should be why do some countries drive on the right?