I wonder if people are right-footed as they are right-handed. Surely even kids with their first bike will get from the left, right-foot first. That also means that you right hand is the first on the controls.
Also, for car-dwelling Americans, getting into something you intend to drive from the left side is automatic.
I recall that the Swedes had a reason for left-hand hinge, right-side entry on the Gripen. I think that it was because the whole cockpit was small, and it meant that you would not get tangled up with the throttle.
Where and how the left-hand entrance (and right-hand service and support) convention for commercial aircraft got established, I have no idea, but it was universal very early on.