Why are flash floods so dangerous?

Because of the sudden flood which, most likely, has not been prepared for.

Flash flood is a rapid flooding of geomorphic low-lying areas: washes, rivers, dry lakes and basins. It may be caused by heavy rain associated with a severe thunderstorm, hurricane, tropical storm, or meltwater from ice or snow flowing over ice sheets or snowfields. Flash floods may occur after the collapse of a natural ice or debris dam, or a human structure such as a man-made dam, as occurred before the Johnstown Flood of 1889. Flash floods are distinguished from regular floods by a timescale of less than six hours.[1] The water that is temporarily available is often used by foliage with rapid germination and short growth cycles, and by specially adapted animal life.