Soylent doesn't fill a separate need from normal food, it fills the desire for a more efficient vehicle of nutrition than normal food. For example, I use Soylent not as an "I'm too lazy too make food" but rather as an easily replicatable nutritional base that is easier to customize to my nutritional needs and preferences than any other option out there that I'm aware of (there may very well be something better out here).
Their success is the result of "magical marketing", but of filling a niche desire of the market. In fact, Soylent didn't even do marketing for the first year or two of its existence. Rinehart just started experimenting, realized it hit a nerve with the hacker community, and created a product that appealed to the demographic that responded to his experiment.