Jedi become one with the force, while Sith control it through sheer force of will. When a Jedi die,they become one with the force. Although Anakin's had redeemed himself at the last minute, his spirit was still too corrupt.
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What about qui gon
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His body didn't disappear because he wasn't fully learned in the technique to achieve "Force ghostiness ". So hi body didn't disappear at that moment. Obi-Wan had a decade or so to become one with the he force before his death.