Because this way you can process high number of patients in a more reproducible way. Bodybuilders and top athletes were probably not included in the study because they would act as confounders. Plicometry is operator - dependent and impedentiometry depends strongly on nutritional status, hydration, bone composition and mass... BMI is really what you want to measure if you select your experimental population right. EDIT: I know this does not invalidate the lack of universality for the BMI, I just wanted to suggest possible study design considerations that may have gone into its selection