To understand why this happens, we need to understand the primary differences in the way the fit of our clothing varies from the fit of a wetsuit, and other differences. Our clothing hangs on our bodies and touches at certain strategic points and makes casual contact at other ones. Shirts, blouses, and jackets hang from our neck and shoulders and usually hang away from our bodies in most other areas. Pants and skirts are drawn about our waist and often hang loose around our legs. There are a few exceptions and some people do wear tight clothing.
Spandex is much like a lycra dive type skin.
Wetsuits differ greatly from this loose hanging concept. A properly fitting wetsuit will make contact over most of the area it covers, leaving as little space as possible between it and your skin. The lesser the space, the less there is for water to enter and carry away your body heat. Spaces where the wetsuit does not follow the contours of your body, such as along one's back, can be filled in with options such as spine pads. Water cannot enter a space that has already been filled.