I dunno the actual history or anything, but I suspect thats why the US put the wheel on the left, because the right arm is usually the strong one, and way back when, I imagine transmissions required a fair bit of force to change gears, before syncromeshing. When I had my Triumph 2000 in the 1980's, I really wished I'd gotten an automatic, because as a manual, that thing was a bitch to change gears with my left hand. And the clutch needed all the strength in my left leg to push.
Lots of people think its because the Brits put the wheel on the right, just to be different, the US put it on the left.
Like I said, I dont actually know the real reason.