Several factors made body soap unsuitable for laundry use:
Soap is made from natural oils such as animal fats or vegetable oils. Soap will react with all the hardness in the water first before it can react with soil in the linen. Therefore, it is effective only for use with soft water i.e. hardness below 50ppm. If water hardness is above 50ppm then detergent is more suitable. Detergent is made from chemicals.
Body soap is usually neutral; laundry soap works better with alkali builder.
Body soap does not dissolve too readily in water, so that it lasts longer. Laundry soap must dissolve as quickly as possible. If there are soap specks which have not dissolved completely during the main wash, they will cause havoc in the washing and finishing processes.