With your cable in your right hand, it is easier to plug into the right side of a pedal. If you were to try to do the equivalent with your left, the guitar neck would be a little bit in the way as well.
But you still have to plug in from the right with the guitar cord, and from the left for the card to the amp. So I dunno, my theory may be bogus. But this pattern got established very early on, by the late 60s st the latest.