The lab was created as part of the nation's balls-to-the-wall effort to build a nuclear bomb with which to smite its enemies in war. I imagine that safety was only a concern to the extent that incidents would slow them down. Otherwise, with thousands of people dying each day in the war, it's worth the risk to people at the labs if they can end the war faster.

Slotin got himself killed after the war was over, but it was less than a year after, and I imagine that sort of "get it done at any cost" culture does not change overnight.

I wonder if that attitude is in fact the root of the trouble today. Especially since I bet that the "save the nation and damn the risks" attitude came back for at least a decade or two once the Cold War ramped up.