From the article:

    Rhinehart is well aware of the fact that Soylent
    isn’t the first beverage designed to replace meals.
    He considered using Ensure for his initial no-eating
    experiment, but found it much too expensive, too sugary,
    too unpalatable, and sub-optimal in its ingredient
    make-up.

    The differentiating factor seems to be in the intended
    purpose. Meal-replacement shakes have never been
    presented as a food — something that you could (or
    might want to) live on for an extended period.