Surface contamination is the reason. Which is why dirty bombs are effective. Surface contamination can be cleaned. This was in fact done at the bikini atoll; what was left anyways. When residents returned they started dying from cancer. Turns out palm trees, and by extension their nuts, absorb considerable radiation from the soil. This meant all trees would have to be replaced too. The US said we're are not spending the money. Regardless, if you don't eat indigenous vegetation at bikini atoll, it's now perfectly safe. But this does confirm a large release of radiation into the environment with a significant loss of land mass.