Exercise is all about movement. Yogasana is all about stability. Ironically, the way yogasana is being practiced is more often in the form of exercise and not the way it should be. Initially this form of practice does help in enhancing flexibility and reducing weight but one is not able to reap deeper benefits. Ideal practice should involve both the forms i.e. beginning with a good warm up and then progressing into asanas.
Exercise works on muscles alone. Yogasana works on all systems and organs of the body. But in this day and age we prefer muscle work out than putting effort in yogasana practice, as the results take time and more internal than external. And who cares about internal?! All we need is to look good. Wish we worked at feeling good as well!
Exercise needs energy. Yogasana gives energy. But if one does yogasana practice in the form of exercise then it is not going to be the same experience. Normally people feel that they are feeling energetic after exercise but that is the adrenaline rush which gets released after work out. It does not stay! Yogasana boosts the overall functioning of all the systems, releases knots which block our energy, eliminates stress from body & mind, thus that is unleashing of energy levels.
Exercise is only at the level of body. Yogasana is at the level of both body and mind. It makes body healthy and mind peaceful. Now this is something which one realises as practice progresses, but one must do it with sincerity and dedication. And then transformation speaks for itself.