The problem with radon is that it is insidious, and it is very good at doing what gas does. It's very good at seeping through a home's foundation, utilizing tiny flaws and microscopic cracks. To make matters worse, it can even permeate through concrete.
However, it would stand to reason that since radon is so good at permeating through foundations and concrete, it should be equally exceptional at escaping from a house. It makes sense, but it is altogether wrong.
With homes becoming more energy-efficient and more tightly sealed up, it does not allow radon the opportunity to escape. Think of a covered pot of water boiling on the stove. With nowhere for the steam to escape, it just builds up. Only in this case, instead of a potato, it's "you" radon is boiling.