Coke products are the only ones with actual extract from the coca plant. It is grandfathered in. They used to contain actual cocaine. They remove that, but they still contain other things from the coca plant.

The very name refers to the coca plant they based the drink on. In contrast, Pepsi and "generic" brands are essentially sugar water. This is likely why Coke had that campaign for a time: "Coke -- the real thing."

People drink coffee primarily for the caffeine. This is generally acknowledged. People joke about being unable to focus because they haven't had their coffee yet, etc.

So, we drink coffee for some meaningful active ingredient. I see no reason to believe we drink soda "for the flavor" and not for the actual active ingredients. Coke has active ingredients that other brands lack.