I belive it's that syrupy thickness people that prefer sugary version like, more than just the sweetness.

Personally, after a long brake from drinking coke/pepsi I can't stand either the original or the zero. I seem to crave the taste sometimes, then I buy one(zero) and it's like "what is this? Why sweetness is literally the only thing that I can taste here?" It feels like they are doubling the sugar in comparison to how I remember it. Most likely it's the same drink, I was just used to the massive sweetener/sugar content before.