Why An insulator is a material whose electrons are tightly bound to their atoms, so very few charge carriers are free to move and current is essentially blocked. Glass, along with ceramic, rubber, plastic and dry wood, fits that description, which is why glass and ceramic are used for antenna standoffs, feedline spreaders and high voltage insulators.
Watch out The other three are all conductors: sea water conducts because dissolved salt provides mobile ions, stainless steel is a metal with free electrons, and graphite is a form of carbon that conducts well enough to be used in resistors and motor brushes.
Insulators: glass, ceramic, rubber, plastic. If it is metal, salty water, or carbon, it conducts.