Speaking as a call maker, it is easier to make a round call because it is done on the lathe and/or drill press. I don't know if that is the main reason, but that is why I do it. You can order slate in any dimension (oval, rectangle, round, etc). There are a lot of slate calls made in different shapes and styles, but I think round is easier to make and is comfortable in your hand. There have also been a few turtles who have made the ultimate sacrifice for the sake of killing toms.