It's not really the radiation from radon itself which is the problem, but rather radon's daughters. Radon is a noble gas, so it's not going to accumulate in you and doesn't get much chance to do damage to people even if they inhale it; they'll just exhale it moments later before barely any of it can decay. But radon's daughters, the chain of atoms which are produced from the decay of radon and each other, aren't gases, so if stagnant air with radon sticks around it's going to 'rain' an atomic dust of radioactive isotopes which can accumulate and, if disturbed, can be inhaled and stick with people.