The displays use electromagnetic technology (flip digits).  This technology uses very little power (important in a battery powered unit), and more importantly, has the highest contrast of any low powered display technology.  The contrast (readability) of these displays is typically double that of an LCD display, and far better than LED in direct sunlight.  In addition, the viewing angle of the electromagnetic displays is almost 180 degrees, compared to as little as 5 degrees for LED which is often blinding head-on and not bright enough to see at a slight angle.