Image by Flickr user r.s.m.b. SeesImage by 20081109-rain-drain.jpg The more concrete and parking lots there are, the fewer places there are for the rain to go.
The area of Charleston west of West Ashley is seeing more and more flooding as its developed. There's a story at The Post and Courier about the flooding in the area after October's torrential rains. Go read it.
The problem is caused by "impervious surfaces" created as asphalt and concrete block the ability of soil to naturally absorb and funnel away rain water. So once you get primed on the concept, head over to Wikipedia to read all about why.