Just read his book. He explains it all. He had health problems while directing his last film. He was having massive headaches on the set. Doctors told him to quit if he wanted to stay healthy. And that's what he did.

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Well, he was 64 when he made POCKETFUL OF MIRACLES. Also, that film's critical and financial failure may have indicated to him that he no longer knew his audience, coupled with the fact that it was not a pleasant shoot. Perhaps the fun of filmmaking had gone out of him, and the decision-making process by tempermental stars and bookkeeping bosses was no longer the Hollywood in which he had flourished so successfully three decades before. He then turned to as a character says in MEET JOHN DOE "writing his memories."
"Take 'em to Missouri"

I had the same question
don't you know there ain't no devil, theres just God when he's drunk