Environment plays a role in determining which developmental adaptations can survive and where. Genetically, if the information for building the collagen isn't there, it won't be built. If the collagen is missing, the bone will be brittle. Any logical answer could be a method of getting the chemical to the tumor. Sharks are cartilaginous fish.
Rats have relatively small amounts of cartilage.
Cartilage needs the movement of fluids for diffusion of nutrients. Fluid is the major component of the hydrotatic skeleton. Muscles act against the fluid. Fluid is part of the cartilage matrix. Fluid fills the synovial joint.