The plausible reason that can be sited is the inherent low calorie property of cabbage.

Calories in Cabbage:

100g = 25 calories

Additionally it has nutritional value as well. Dark green leafy cabbage is high in vitamin C as well as iron which make it a perfect food for anyone suffering from anemia as well as pregnant women due to its high foliate content. Cabbage also contains beta-carotene and potassium. As a member of the brassica family, it also contains phyto-chemicals like glucosinolates which are proven to reduce cancer, especially lung cancer.

Claims made for the cabbage soup diet range from the ludicrous, ie. the cabbage soup itself has fat burning properties to the dubious, ie. you can lose 10lbs in a week. The diet is often put forward as a quick way to drop pounds for a special event and most proponents advise that it only be followed for a week.