Why isn’t Hooke’s law a universal law?

Because it is not valid under all conditions. In science and physics, a universal law of nature is one that holds true in all conditions and at all times. Hooke’s law has limits in that it can only accurately describe results when the force and level of deformation are small and within the proportionality limit of a given material. In addition, not all objects and materials can be described by Hooke’s law, it only pertains to those with elastic potential.