I guess it's because being extroverted is more positive for society as a whole than being introverted. And so society is trying to -- even if it's not always a conscious efforts made -- kind of encourage and reward this type of behaviour.

EDIT: of course it's not all that simple. IMHO a society of pure extroverts would be an unstable network of salespeople with nothing to sell and no one to engineer and manufacture the things needed. I'm joking here, but... :)