One reason is that the artificial sweeteners in diet sodas create a negative hormonal response in the body that increases fat storing hormone production and increases cravings for more sweets and refined carbohydrates in the time period after consuming the diet drink.
It's not a good idea to try to "fool" your body with artificial sweeteners. Essentially you "fool" your body by drinking something very sweet (sweetened with artificial sweeteners), but you don't actually ingest the sugar calories your body expects. Your body has a hard time knowing how to deal with that.
With artificial sweeteners, your taste buds "sense" that you ingested something very sweet, and even though you might not have ingested sugar, your body follows with an insulin response. As you know, higher insulin levels can increase cravings as well as triggering your body to deposit body fat. This is not to say it's any better to drink a sugar-sweetened soda which has it's own host of problems.
People laugh at bodybuilders with our jugs of water, but hey there's something to be said about the goodness of water.