Because most popular styles of Yoga are "Yang" in nature. They emphasize muscular effort and repetition. Yang activity is an essential form of exercise if you live a sedentary lifestyle. Yang exercise energizes us, throws off fatigue and overcomes depression. But it is only one half of the equation. We also need to learn to relax, be patient, be quiet and focus on the skeleton and its joints. I emphasize Yin in my book and my workshops because the Yang styles of Yoga are well represented in every corner of the country.