Look at a simple single - component system. And PT diagram for this system. Suppose that supstance we looking at has three phases solid, liquid and gas. In that case we have critical point between liquid and gas phase, but not between solid and liquid phase. Why? Why solid - liquid coexistence curve doesn't end in critical point? This is because solid has crystalline symmetry. In other hand liquid is homogeneous and isotropic. So liquid has much greater degree of symmetry than solid. What do you think about paper which I attach in the post. This is simulation, not experiment. I think that this is interesting to discuss!