A home built over a crawlspace, especially one with a dirt floor, sucks up water vapor from the earth and allows it to enter the home.
Crawlspaces were built to provide a buffer between the house and the damp, wet earth. Seemed like a good idea at the time.
They were also a convenient place to put piping for domestic water, electric cables, and the ductwork used to circulate conditioned air throughout the house.
Builders and homeowners alike (if they bothered to look) could see that crawlspaces were places where water would collect, and creepy crawly things thrived.
They also realized that living one floor up from a humid, swampy environment was not ideal.
Family members wheezed, closets began to grow mildew, musty smells invaded the home, and sills and framing got chewed up by termites.