The night fission was discovered

The New Yorker has a fabulous short piece titled "Fateful Night" by Eugene Kinkead about the night of January 25, 1939 "when Dr. John R. Dunning, the physicist, split the uranium atom at Columbia University and realized that it was possible to release atomic energy."

In this recreation of the events of that evening and night, the main characters come alive; I guess this is the most important advantage with this literary genre. After confirming the key observations of their experiment and ruling out some of the competing explanations, Dunning and his colleagues knew they had a major discovery on their hands. What happened next?

The three men pledged themselves to secrecy off the campus until other members of the faculty had had an opportunity to corroborate what they had seen.

Dunning gets back to his office to do some calculations to estimate the amount of energy that's given off during the splitting of uranium, and Kinkead's story ends there. But the line quoted above made a strong impression in my mind. I wonder why ...