"They" can make lenses longer than 210mm for medium format. Bronica has lenses up to 500mm, Fuji has a 500mm for the GX680, Hasselblad has a 250, 350, and 500mm lenses, Mamiya has a 500mm for the RB/RZ series, Pentax makes a 500, 600, 800, and 1000mm lenses for the Pentax 67. You'll notice, however, that all of these cameras are SLR and provide focusing through the lens itself on a viewfinder screen and not through a range finder mechanism.
If you're talking specifically about the Mamiya 7, I could only guess at the answer. I would think making the range finder work with telephoto image magnification would be very difficult.
The range finder has two windows, and using the parallax between the two windows provides a focus mechanism. The windows probably cannot accomodate the narrow angle of telephoto lenses making it impossible to show when the lens is in focus (much like really wide angle lenses require an auxillary viewfinder for the same reason). Because of the shallow depth of field with longer focal length lenses, this would be a critical issue.