I think this is a very good question. Designers have adopted extraordinary measures to reduce the effective mass of the moving elements of a cartridge. The primary advantage of moving coil over moving magnet is that the effective mass of coils is lower. The coils are located close to the pivot point of the system, so any given amount of mass at that location has FAR less impact on the effective mass than the same amount of mass located at the tip of the cantilever. Thus, the mass of the cantilever, particularly near the type, matters a lot. That is why ultra light metals formed into a thin tube is employed. This minimizes mass for any given stiffness of the assembly. A solid diamond cantilever would maximize stiffness, but it would weight quite a lot even if the diameter is made quite small. Unlike a tube, a solid rod does not maximize rigidity for any give mass. The reason to use diamond is to have extreme rigidity and as little flexing or dissipation of the movement of the stylus as possible (all of the movement transferred to the generating element (coils or magnet). You do NOT want the cantilever to do any kind of damping or any other alteration of the movement of the cantilever; the suspension located after the generating elements do that job.
I suspect that solid diamond cantilevers have higher effective mass, but, the extreme rigidity makes the tradeoff worth it to the designer.