I was surprised to read that the reason isn't that green olives are typically used to make martinis. It seems to me that having a jar of olives on a bar would make for a better appearance than a can. Also, green olives are typically stuffed with pimentos or garlic or blue cheese, and so being able to see them might help sell them, whereas black olives are used for things other canned vegetables are used for, ie salads, casseroles, etc.