@MeBigbrain @Terry
Looked this up quickly, and mostly 2 things made this possible.
1) The bombs exploded while still very high in the air. The damage was mostly caused by the shock wave and extreme heat, but the radioactive elements themselves were quickly dissipated by the winds. They didn't have time to land on soil and concentrate in those areas.
2) They were first generation bombs, that were actually quite inefficient, and didn't produce a lot of the really radioactive elements.