Sunlight is much more natural as well... Corals get uniformly lit by the sun as it crosses the sky during the day. Pretty hard to do that with our current tank lighting systems, unless your willing do a motorized system, so you get hot spots or dead spots of light which directly affect coral growth.
Can't forget, Mariculture facilities may also "prune" corals as they grow to give them uniform growth pattern. That's just a guess though.
