Why would wearing several layers of clothing keep you warmer than wearing one heavy coat?

Um there is a yes and a no

Wearing several layers of clothing can keep you alot warmer then just one heavy coat because with layering your wearing thin pieces and maybe a few thick layers of shirts. You'll stay warm longer because it'll have more heat to it. The one heavy coat won't keep you that warm because it doesn't contain heat for long when your in cold weather. The jacket will let you breath but you'll feel the air when you don't want to. Hope this helps.