star wars - Why do some Jedi disappear when they die while others don't?

Some Jedi disappear when they die:

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  In Star Wars Episode IV: A New Hope

  
  

  
  

  At the end of his duel with Vader on the first Death Star, Obi-Wan Kenobi let down his guard and concentrated for a moment, disappearing entirely just as Vader's lightsaber passed through his empty robe.

  
  

  
  


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  In Star Wars Episode VI: Return of the Jedi

  
  

  
  
  

  When Luke returned to Dagobah in the Return of the Jedi, Yoda also disappears when he peacefully dies.

  
  

  
  


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  In Star Wars Episode VIII: The Last Jedi

  
  

  
  

  When Luke is exhausted after “being” on Crait he looks into the Horizon and disappears.

  
  

  
  

And others don't:

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  Star Wars Episode I: The Phantom Menace

  
  

  
  

  When Qui-Gon Jinn's dies after his duel with Darth Maul, he doesn't disappear and is cremated.

  
  

  
  


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  Star Wars Episode VI: Return of the Jedi

  
  
  

  
  

  When Vader/Anakin Skywalker dies on the second Death Star, he does not disappear and he is cremated by Luke.

  
  

  
  

Also, they all became a "ghost" after their death.

Answer

In the Expanded Universe novels it is shown that most Jedi do NOT disappear upon death. Those who do have 'become One with the Force'.

It is implied that only those who disappear can re-appear as Force Ghosts, but Vader seems to invalidate this (EDIT: Correction - Vader's organic body DID disappear, what was burned was just his suit - see the section on his death, near the bottom.). A better theory is that learning how to become a Force Ghost - a technique Qui-Gonn is said to have rediscovered enables a Jedi who is prepared for death to disappear upon their physical form dying.

To disappear upon death requires great strength in the Force (equivalent to a Jedi Master), great wisdom and understanding of its ways, and for the person who dies to be at peace.