There are actually a few very flat lenses for SLR cameras like the Nikon e series 50mm lens.
But as a general rule, making a flat lens for a reflex is pretty hard because of the registration distance that is pretty large. Most SLR lenses need a reverse telephoto design that forces the lens to be somewhat big. On the other hand, rangefinders are on the flat side so you don't need any special design to get them to fit.
And to make things better, ms optical sometimes uses levers instead of rings which makes it possible to set the lens well inside the camera, this is something no other manufacturer that I know has ever done.