I drink Coke Zero, too. The other day I learned from a FA some yet untold story (in this thread).
CokeZero is a real pain when opening the can/bottle at cruise level (underpressure), even
worse than beer. If bottles, some FA might even prepare the flight by opening and closing bottles
prior to takeoff, which obviously wont go with cans.
So maybe the sentence in FA's head went like: "no, not this spilling again" (and that's why you
wont get a closed can).