as wolf says, despite all the ads featuring office girls watching hunky construction workers quaffing Diet Coke, it was actually the typists, not the builders, who were buying it. So they had to come up with a Bloke Coke to serve the other half of the population, and Zero is it. Very different flavours. Diet Coke has citric acid and Coke Zero has sodium citrate.
RIP Tab, as featured in Back to the Future. (Though apparently you can still buy it in the USA.)