Because from an anthropological evolutionary introverts are seen as more passive, and of less benefit to the tribe of primates (and we are an aggressive primate, the human). It is a evolutionary hardwire. On the flip side, extroverts and alpha males and females will be respected by other primates, as they may be able to provide protection in times of crisis.
This is hard to digest. Ugly. but such is life. (it begins in the playground, this behaviour, and it never ends).
Have a great day!
Tony