Looking at the history of Yoga, over time the practices had to become more and more detailed or refined in order to address our own escalating sense of alienation from life. The earliest writings about happiness, realization, living in harmony, knowing God and knowing Self seem to us now very idealistic, very philosophical and very difficult for people in the present time to understand and put into practice. For example, The Rig Veda teaches, “Who knows this? No one knows.” Most of us needed a little more direction than that. So the teachings of the Vedas were distilled in The Upanishads and are presented as narratives, stories and parables. 
Many people still needed more concrete instruction than that, and eventually we got The Yoga Sutra and The Bhagavad Gita, much of which we can relate to even now, but much of which still strikes us as abstract. Then in the Middle Ages came The Hatha Yoga Pradipika, which gives detailed instruction for fifteen asanas, among many other practices. (source- excerpts from:for jivamuktiyoga.com)
