@RikerGoogling still love that the true answer to this is that Wizards (the publisher) originally thought expansions to the game would be totally standalone and so they'd change the card backs with each new set. Or in other words, it was a foolish mistake they've had to live with ever since - and that's beautiful to me.
@RikerGoogling Because it is traditionally played on board a full rigged Ship of the Line, and they have to get the Deckmaster's permission to clear the sailors and begin the game.