This question originally appeared on Quora. 
By Gayle Laakmann McDowell, Author of Cracking the Coding Interview and The Google Resume
Because humor is typically sexual, crass, inappropriate, offensive, racist, and/or mean, and women are supposed to "play nice" with others.
Don't believe me? Look at comedians' jokes:
- Racist and sexual: "You know the world is going crazy when the best rapper is a white guy, the best golfer is a black guy, the tallest guy in the NBA is Chinese, the Swiss hold the America's Cup, France is accusing the U.S. of arrogance, Germany doesn't want to go to war, and the three most powerful men in America are named 'Bush,' 'Dick,' and 'Colon.' Need I say more?" [Chris Rock]
- Sexual / crude + sexist: "Chivalry died when women started readin' the shit in all them magazines. They got too much advice about men from other women. And they don't know what the fuck they're talkin' about. I see them in the grocery store, says on the cover "100 Ways to Please Your Man" by some lady. Come on, man. Ain't no 100 ways. That list is four things long. Just suck his dick, play with his balls, fix him a sandwich, and don't talk so much and he'll be happy!" [Dave Chappelle]
- Sexist and mean: "Here's a little known fact - Arnold is the first body builder to run for governor since Janet Reno." [David Letterman]
Growing up, little girls are encouraged to "play nice" with others. Smile at and compliment your friends; teasing them is mean! Wait your turn to speak; you don't want to dominate the conversation! Sexual comments aren't classy; in fact, don't even talk about sex. You don't want to be one of those girls. Don't be aggressive and independent; people like "team players." Please don't draw so much attention to yourself; just play within the lines, okay? And please, whatever you do, be nice to and respectful of others.
When those little girls grow up, all the things that make people funny have been socialized out of them. They couldn't get away with humor as a kid and, in many cases, they still can't as an adult. (A guy can get up on stage and talk about the four chicks he banged last weekend, and people will think more highly of him. You think a woman is going to get up on stage and talk about the four guys she slept with?)
The good news is that this is changing. There's less pressure to make girls be meek and timid, and more leeway for girls and women to be funny in the ways that boys and men have been. And it shows -- there are some hilarious female comedians!