Rereading The Making of the Atomic Bomb this week it struck me that the Vice President knew nearly nothing about a military secret the size of the American auto industry until FDR suddenly died. He was briefed by Secretary of War Henry Stimson, which makes sense; but also by James Francis Byrnes, a powerful politician who had held many posts, but who by early April was a private citizen. In a time of great consequence, why did FDR handicap the VP by keeping him in the dark about the development that would define international relations for decades to come?
We don't know. Roosevelt is in many ways a mystery. He did not confide to many people, he kept no diaries, he justified his actions rarely, and he kept secrets habitually. He didn't like Truman much — that much is clear — and he doesn't seem to have trusted him much. Still, given that his own health was never a foregone conclusion (and indeed, Truman had been picked as his VP on the assumption he would succeed Roosevelt), keeping him out of the loop on something of this consequence can only look like negligence. By contrast, his VP before Truman, Henry Wallace, knew much about the Manhattan Project and was involved on the Top Policy Group of the project.
Stimson did not like Truman much, either, and Truman had many times (not just the once that is well-known) attempted to audit the project while he was a Senator. So perhaps that played a role in FDR's distrust. But ultimately, this is one of the many things about how Roosevelt managed his affairs that we do not have any kind of good answer for.