Question

the substances charged by friction are mostly insulators why??

Answer 1

Static electricity generated by rubbing two nonmagnetic objects (insulators) together.
The friction between the two objects generates attraction because the substance with an excess of electrons transfers them to the positively-charged substance.
Insulators do not conduct electricity and are good at holding a charge. These substances may include rubber, plastic, glass or pitch.
The electrons that are transferred are stored on the surface of an object.