You need to read the accounts of the bombings and the resulting injuries and the incredible benefit it would have been to the Japanese to know duck and cover.
“Duck and cover” made sense in that place in that time—particularly to avoid blindness and burns from a nuclear blast.
Remember, though, that the strategy must be implemented in a very few seconds.
Now imagine you are driving and stuck in midtown Manhattan traffic (better yet on FDR drive!).
If you “duck and cover” you will become part of a massive auto accident—best case.