This is one of the most common questions we get, along with its follow-on: why am I just now hearing about radon (ie, is this some sort of modern scam or what)?
The answers to both questions are pretty straightforward. Radon seeps up through the soil in its natural life cycle. It is a gas made of tiny particles, and as it decays it moves up and into the atmosphere. Our homes have a negative pressure caused by our heating and air conditioning systems. Think of it as a very low pressure vacuum cleaner, which leads to a suction that gathers more of the earth gases into the home than outside of the home. As I mentioned, it is a very, very small pressure difference caused by the heating and cooling system, but it nevertheless is a difference, and that means more earth gases (and hence radon particles) enter your home.