Question

why are p-block elements known as representative elements???

Answer 1

Not only p-block even s-block elements are representative elements.

For both s&p block ,their outer shells are not completely filled with electrons .They follow normal rules for filling the orbitals.
 
These  s&p block elements get nearest inert gas configuration by lose or gain of electrons.

In s&p block all the elements down the group have a of having sane number of electrons in their outer most shell & represent same electronic configuration . Hence s&p block elements are representative elements