My understanding was more that Coke Zero was originally targeted at men, who apparently viewed "diet" sodas as being for women. [0]
"We're positioning Coke Zero as a defender and celebrator of guy enjoyment."
It doesn't hurt that it's a chance to reformulate without people complaining about the change like "New Coke" (which was allegedly based on the Diet Coke of the time[1]) and so the Zero branding has likely expanded across the Coca-Cola lineup and other brands to expand that reach and introduce changes - and incidentally give them a chance to mess with their agreements with bottlers the same way as when they switched to high-fructose corn syrup [2] [3].
I don't find that weird - if I'm going to be drinking a coke (of whatever flavor), caffeine-free Diet Coke is my preference over regular/Zero. It is a different formula, even if it only comes down to the blend of sweeteners. Obviously I don't know the actual formulas, but each has a different blend of sweeteners based on the labels, and now I've added "The Real Thing" to my reading list. Different people prefer different blends, like the other commenters who prefer monkfruit, erythritol or stevia.