The Real Thing?

Saturday morning we pulled a 2 liter bottle of Coke out of a cooler and inadvertently left it out on the back porch.  It's been cold here.  Also left outside were a bottle of Sprite (2 liter) and a quart of tonic.  By Noon on Sunday, when it was fairly cold here in Philly, the Sprite was frozen half way through, the tonic was frozen solid and the Coke had no signs of freezing.

This morning, still way below freezing here -- the Coke was completely liquid.  I find that both disturbing and interesting.  Why is it that Coke won't freeze?  Anyone know?