It's actually the inverse - in the last 40 years this spill has been radiating away and getting less dangerous. However, for that period people have been exposed to it, and it's been sitting there as an accident waiting to happen if (for example) the site gets flooded.

You did not read the article, you did not read my post, and you didn't read yours, for that matter.

My guess, with a certain US Political Party cutting funding for things like the EPA and Super Fund Activities, I would guess no one had the funds to properly diagnose and do the appropriate cleanup.

So they decided it was good enough for "now" until this was discovered. I would say the Company that caused the spill will get away without paying cleanup costs. That is what usually happens with these type of Environmental Damage in the US.

There is also a risk to disturbing the site. Sometimes leaving something alone is the best option.