Solid food is better in terms of nutrition, more filling and it burns calories digesting. However, it's also much more time consuming and harder to measure accurately. Liquid foods are easy to prepare and easily measurable. V-diet is a very extreme diet, but one of the things it relies on is that it's very simple.
You don't think about food, how to break it down, how to prepare it, etc... you don't get the opportunity to eat that extra portion and you can't measure it wrong. You drink your shake, the end.