While some believe that this is because its demo is too close to that of Kick-Ass, which underperformed based on expectations, but Deadline says that's not the case: "Marvel simply has too much going on at the moment and will take its time on the live action adaptation of the comic hatched by Lost writer Brian K. Vaughan. It's called the Breakfast Club of superhero saga, dealing with the children of super-villains who are cursed with evil superpowers. They live in a secret lair under the La Brea tar pits, and try to use their unusual powers for heroic purposes."
Although director Peter Sollett is still attached for whenever the film does happen, it's been put to him that there would be time for him to do a different film while waiting for The Runaways to ramp up.