Sorry, pet peeve: its main ingredient is dextrose, a sugar, which is a carbohydrate, which contains 3.87 calories per gram. Yet the manufacturer claims zero calories due to rounding error. It also has maltodextrin; a carb. Only the sucralose part is a diet product.
Because some taste better than others. They don’t all taste alike. Diet Coke is similar to the discontinued New Coke. Coke Zero is more like Coke Classic. My personal preference is for Coke Vanilla Zero.