Great question! The natural sugars in Coca-Cola add flavor and somehow work to tenderize the meat. You'd have to get deep into food chemistry why this exactly works, but it does! I use the same concept in my Slow Cooker Cherry Coke Pulled Pork and it works like a charm. Please note, you need to use regular, not diet, Coca-Cola. If you can find Mexican Coke at a place like BevMo or even Home Depot of all places, it has a bit more sugar and the carnitas are just that much better. But if you don't have it, regular Coke is just fine.