The answer comes from another PBS series on the Venona papers. The national security agency had broken the Russian code partly during the 1940's. (As they had the japanese and german code.)What they found out was that the soviets had spies through out FDR's administration and also in the a bomb project. In addition to the spies apprehended like Julius Rosenburg who had worked on the a bomb project--the NSA read russian cable traffic which showed there were three more spies on the project. The feds couldn't tell who they were. However, from reading the cables they learned that the soviets recruited mostly through the communist party. Oppenheimer had been a party member at one time.
There is no excuse. Those who wanted to know, knew about the murder of millions in the Soviet Union. Those who associated with the CPUSA knew better than anyone. But they didn't care. Christians were being murdered. Vasily Grossman participated in the Ukrainian Holocaust as a "party activist". In later life, he had some regrets (he never saw how planned out it all was however). Read his comments HERE.
From Brighter Than a Thousand Suns, by Robert Jungk:
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Oppenheimer was convinced of Chevalier's innocence. But (Oppenheimer) gave in, and at last disclosed the name of the man whom he had himself, as he admitted later, so deeply incriminated...so saving himself and his career. Chevalier knew nothing of it, but shortly afterwards was dismissed from his tutorial post for unknown reasons. It was not until more than ten years later, after he had been driven into exile and was still without an appointment, that he finally learned who had informed against him and brought his academic career to an end.
In 1954, in an official hearing, Oppenheimer admitted that his stories about a mysterious intermediary, whom he subsequently named to Groves as Chevalier, had been "idiotic" and "a tissue of lies."
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This, from the man who was such a left-wing martyr for losing his own clearance.