Why do some dirt jump riders only have a rear brake?

Running only a rear brake eliminates the front brake cable that would wrap around the stem during bar spins and tail whips. It's a trick-oriented choice—bar spins and tail whips are core tricks in dirt jump and slopestyle, and a front brake cable gets in the way. If you're not doing bar spins or tail whips, keep the front brake for safety. Many riders start with both brakes and remove the front only when they begin learning rotational tricks.