But don't be too quick to dismiss masculinity or coolness factors in product selection. Diet Coke and Coke Zero are essentially the same thing made by the same company (the latter having a slightly different "flavor profile"). Coke Zero exists because some men or many men will not order a Diet Coke: it's sissy or uncool. Coke Zero was introduced solely for that reason.
Compare to e.g. Diet Pepsi, which is to Pepsi what Coke Zero is to Coke, Coca Cola was for decades worried about making something that was too close to the original (hence Tab being their first diet cola)
Or Diet RC Cola, which is the only diet cola product I've ever been close to confuse with the real thing (my parents insisted I "wouldn't notice the difference" with diet colas as a child in an attempt to get me to accept them, and utterly failed).
If you want a diet cola that tastes like a regular cola, Diet Coke will never be it, and it left Coca Cola with a gap in their lineup for people who weren't as concerned about picking a diet product, but would pick one if there's a diet cola available that is "close enough".
[1]: http://www.independent.co.uk/news/media/introducing-bloke-co...
It's as simple as marketing, which does matter. Slimfast in it's name implies a diet drink, not food replacement. Most people would never even consider them in the same arena and that's due to simple marketing by the very companies that produce them.