The science of grilling the perfect steak
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.
Your choice of fuel also matters. Gas grills are great at fast heat response and maintaining low temperatures for a wide range of proteins and vegetables, but charcoal can burn hotter for better crust and has compounds like guaiacol and syringol that add a warm complexity to your steak.
You'll notice the effect most clearly if you stick to natural chunk charcoal as opposed to briquettes (it burns hotter and cleaner for more wood smoke flavor and less gunk) and skip the lighter fluid (which makes your entire barbecue smell like gasoline) for a classic chimney starter. Happy grilling.