When artists painted a person’s portrait, they often arranged the canvas so that was taller than it was wide, in order to focus the composition on the person’s face and body, which most commonly have a vertical orientation. But landscapes were usually painted on canvas that was wider than it was tall, in order to capture the panoramic feel of looking around a broader scene.
Though vertical and horizontal are also perfectly normal ways of referring to these to types of rectangular compositions, the artistic jargon has established itself in most contexts. One major exception is “vertical video.”