Coaxial cables consist of the main conductor which is insulated by a dielectric insulator followed by a braided metallic sheath and the outer wrap.
The reason for coaxial cable to be highly insulated is to ensure the safety of humans and to minimize the loss of energy because strong insulation offers greater protection against energy leakage.
Another reason for insulating them is that they emit electromagnetic and electrical energy in the form of heat therefore high insulation prevents the coaxial cable from doing so