Every asana involves some or the other action. But the asana where you don’t do any action is called as Shavasana. There are three important components of Human existence, these are body organs, energy supply and mental activity. In Shavasana, one has to stop or reduce the activity on all these three levels. He has to stop or reduce his physical activity, reduce the energy requirement by reducing the strain and also reduce the mental activities.
Stopping the physical activity is somewhat easier but stopping the mental activity is extremely difficult. Hence the Shavasana is difficult.