It strikes me that this is the kind of thing an obsessed hobbyist could make for himself. The only difficulty being getting the gantry rigid - a triangular section box structure might be best, and to make the hinge have no play in it. Possibly even cannibalize a 3D printer or a laser cutter using the same mechanism. Much to the horror of everyone I suspect a lot of it could even be made on a 3D printer to prove the concept, even if plastic isn't the ideal for the best longevity.
So, next question, is there a good reason why this design hasn't caught on? I imagine it is the lack of forgiveness in the motor driving the cartridge traveller along the gantry. As I understand it, a linear tracker has a little freedom in it's little tonearm to move left and right to compensate for imperfections in the arm drive (but in so doing, it loses some of its vaunted tracking accuracy). If there were a motor driving the cartridge, it would not only have to adjust the rate of travel as it crossed, it would have to detect and speed up for the spaces between tracks. That means an optical sensor riding just inboard of the cartridge mounting.