I know the answer is “marketing” but I find it really weird these companies are making second drink lines alongside long successful Diet lines. Coke Zero seems like the culprit here. It became a billion dollar brand by fulfilling the same calorie-free needs of customers (especially men), without the stigmas of Diet Coke. Then in the last couple years, both Pepsi and Coke decided to shift drinks to “Zero Sugar” and “Zero” branding more widely. Sprite and Fanta are often found with Zero now and their Diet brand versions have mostly been replaced. But that’s not the rule.
So now we have Pepsi Zero Sugar (originally Pepsi Max) and Diet Pepsi, and then Mountain Dew Zero Sugar and Diet Mountain Dew.
Some people think there’s major differences in ingredients, but there’s typically almost no differences there. Flavor does differ a bit, with many saying drinks like Coke Zero Sugar have a stronger and “less diet” taste than the legacy Diet brands.