theDecline said:
So how come when Yoda and Obi Wan die there bodys disappear but when Vader and Qui-Gon die there bodys need to be burned?
Lucas actually addresses this in ROTS. At the end of the film Yoda informs a shocked Obi-Wan that he’s been in communion with Qui-Gon and that he has new training for him. This deleted scene from the script sheds some light on it:
YODA: Failed to stop the Sith Lord, I have. Still much to learn, there is …
QUI-GON: (V.O.) Patience. You will have time. I did not. When I became one with the Force I made a great discovery. With my training, you will be able to merge with the Force at will. Your physical self will fade away, but you will still retain your consciousness. You will become more powerful than any Sith.
YODA: Eternal consciousness.
QUI-GON: (V.O.) The ability to defy oblivion can be achieved, but only for oneself. It was accomplished by a Shaman of the Whills. It is a state acquired through compassion, not greed.
YODA: . . . to become one with the Force, and influence still have . . . A power greater than all, it is.
QUI-GON: (V.O.) You will learn to let go of everything. No attachment, no thought of self. No physical self.
YODA: A great Jedi Master, you have become, Qui-Gon Jinn. Your apprentice I gratefully become.
Yoda and Obi-Wan spent years training and when they were ready to die let go of their physical forms. Qui-Gon didn’t discover this until after he passed on. Anakin never knew of this power and was likely helped to come back by Yoda and Obi-Wan.