Due to the differences in air pressures,
vented crawlspaces suck air in from outside.
Thus, the floor above it has a negative pressure, so it sucks the air in the crawlspace up into the living quarters, bringing with it everything you would expect to find in a dark, damp air-swamp including Radon.
1/2 of every breath you take in your home is filled with crawlspace air and you spend 70% of your life in your home.
Clean it out, Seal it tight, and Dry it out.