diet coke vs. coke zero

Have you, yourself, found [yourself] face to face, at a McDonald's, wondering why on earth Coke A Cola would spend the money on having two different types of diet colas? or why haven't they phased out the old finally..

I, myself, have pleaded with the lady at Subway to disclose with me the difference and after many futile attempts i began to wonder: 

"am i stammering upon some ever-lasting gobstopper grounds? will Slugworth confront me in a dark alley and tell me to stop putting my nose where it doesnt belong?"

I pressed forward. 

i pulled together many different answers from the web and this is what i suppose is the best answer:

1. diet Coke is not made from the original formula Coke a Cola is made from, also artificial sweetener.

2. on an advertising hand, men are more likely to buy something "out there!" rather than a safe diet soda. Coke Zero sounds more neutral, thus more appealing to men. 

           b. also Coke Zero is, in fact, made from the original formula and is                  said to taste more like real Coke. (THIS, HOPEFULLY, MEANS THE              ORIGINAL FORMULA THAT SUPPOSEDLY HAD COCAINE, YA!)