The science comes down to two key points. For one, when you're grilling meat above coals, fat renders out and falls down into the fire, where it burns away and turns to smoke. This smoke then rises to surround your meat. Smoke particles are particularly sticky, so once they adhere to your meat, they stay there, adding a flavor that's, well, smoky. No matter how well-seasoned your cast iron pan may be, you'll never get quite the same effect. Cast iron produces exceptional browning, but without that vertical distance from the source of heat, smoke can't emerge in the same way.