Successful marketing! The difference imo is that Diet is marketed to women and Zero to men - something which I have noticed but obviously cannot offer 'objective evidence' for. In Greece at least, the Diet/Light variety was generally considered more effeminate - cursive writing, the fact it used to be women that concerned themselves with watching their calorie intake etc. Times are a-changing ofcourse and the red on black, "bro-coke" is here to stay!
I personally prefer bland carbonated mineral water -
I have to watch my figure after all!
