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.
Typical modern practice would be to have a pedal board with all interconnects and power set up and fixed. Hendrix didn’t do that, but EVH did. And now the internet is full of people posting their pedalboards (P&W people especially).
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.
