Why aren't 50% of us left handed?

Sorry if this was answered before. I will be happy with a full, real answer or like I am 5.

Edit: Wow, thanks for all the responses!

I think the tl;dr is: lefty cavemen were too busy fappy to learn how to use new tools, athletes are the best sample of our peak, and religion suppresses equality. So just remember, shake with your fap hand, not your wiper.