Personally, whenever I take personality tests, I usually come up with a dead even split between introvert and extrovert. So I can empathize with both sides to a degree.
Someone threw out a statistic that something like a quarter of people are introverts. That definitely makes sense why they would be in the minority.
A) Until we developed guns, we weren't really the biggest badasses on the food chain. Although, intelligence is on our side, the ability to work together as a group inteligently greatly increases our chances of survival. Without a gun, one guy is going to have a lot harder time going head to head with a pissed off carnivore, than a guy that has all his buddies with him.
B) Ummmm...it's a lot easier to breed when you know how to go out there and meet people. In the modern internet era, yes, there are ways of communicating that are less taxing on an introverts sense of self, but before that, to get laid you got to be able to go out there and meet people. And extroverts are out there more, meeting more people and presumably getting laid more. Hey, they like the attention!
C) Even among our own species, being an introvert with a wild idea could get you killed once upon a time. "Hey guys, you know the church may not be right about xyz thing." And if you don't have buddies to back you up, poof you're out of the picture, real quick.
I dunno, just some ramblings that crossed my mind. Maybe I'm totally wrong...