Answer: Because they get past white and move on to the black belt.
The joke is a pun on the definition of the term "hipster." In the context of the joke, a hipster is someone who pretends to be interested in obscure or niche topics in order to appear cool or trendy. The punchline of the joke is based on the fact that the term "hipster" is often associated with the color white, as in "white hipster." By saying that hipsters do karate and move on to the black belt, the joke suggests that hipsters are no longer interested in being white, or mainstream, and are now interested in something more obscure and niche, like karate. This unexpected twist on the definition of hipster is what makes the joke funny.