Because your mother has a dirty whorish mouth...  Just kidding.  It's just one of those things.  The age group is a big factor.  If you are in your teens, twenties, or thirties you grew up with momma jokes being the ultimate insult.  Whenever you wanted to really piss someone off, you went with the momma joke.  It's pretty lame.  We are online and no one knows who anyone is in real life, so the jokes don't even make think twice.  But the kids will keep doing it whether or not they get the response they want.