The main reason is, it would be hard to frame properly as the user of such a camera, and it is probably hard to design a rangefinder that would track accurately enough to a long lens. The longer the lens is, the more any slight error will be magnified, along with the parallax that all rangefinders have to some degree. Then, how would you set the rangefinder optics to match a wide range of lenses? It probably would be difficult and expensive. For most types of shooting, a long lens is not needed. For long lenses, an SLR is still the better choice since you are looking through the lens and what you see is what you get (more or less).