Because the rock formations in some areas are more prone to production of Radon gas which slowly diffuses through the soil. Basements in those areas tend to have Radon infiltration. It is obviously very slow but the basement is a low point and often not well ventilated which can allow the Radon to accumulate.

Poured slabs are less permeable, not a low point, and tend to have better ventilation either intentionally or unintentionally.