Coke Zero exists because they decided to try to make a diet version of Coke that actually tastes closer to regular Coke. Diet Coke does not use the regular Coca Cola recipe with sugar replaced, but a tweaked one, on purpose.

Compare to e.g. Diet Pepsi, which is to Pepsi what Coke Zero is to Coke, Coca Cola was for decades worried about making something that was too close to the original (hence Tab being their first diet cola)

Or Diet RC Cola, which is the only diet cola product I've ever been close to confuse with the real thing (my parents insisted I "wouldn't notice the difference" with diet colas as a child in an attempt to get me to accept them, and utterly failed).

If you want a diet cola that tastes like a regular cola, Diet Coke will never be it, and it left Coca Cola with a gap in their lineup for people who weren't as concerned about picking a diet product, but would pick one if there's a diet cola available that is "close enough".