One issue is Diet Coke is vegan, diet Pepsi isn’t so can affect people with dietary preferences/requirements.I've said this before, but when I ask for a "Diet Coke" in a restaurant/pub I just mean it generically. So when they ask: "is Diet Pepsi OK?" I'm fine with it.
In fact, I think Diet Coke, Coke Zero, Pepsi Max and Diet Pepsi all taste the same. All are fine with me!
Every Pepsi based drink is also foul