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... :)
No one says they are not needed. It's just that extroverted individuals typically create more relationships within society, and those relationships are usually stronger. However, it's not always the case -- for example, introverted individuals may have a smaller number of relationships, but those relationships may be much stronger.
Society exists because of those relationships, and so it is good for its survival and success if the number of relationships or connections within society is growing and they are becoming stronger. It is basically the existential need for any society.
(I'm sorry if I'm wrong, I'm not a specialist in this field, it's just a bunch of guesses based on my observations)
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... :)