[Not just the island they'd probably break all of reality with that much bad luck. Now watch as I wiki-fu a plausible answer that's still probably bullshit anyways.]

Well, I guess they aren't really, but they've always been a symbol of bravery and courage. [Just look at Richard the Lionhearted right... though for continuity's sake he probably came after King Arthur IDEFK.] A lion was also the faithful companion of one of my knights once.