The idea of a piggyback carrier has been around for a long time (NASA even originally planned to use it; the Shuttles were originally intended to launch from the back of a 747 at cruising altitude) but actually getting it to work, with the clearance needed for safe detachment, has always been difficult. It was cut from the shuttle program IIRC due to budget constraints; NASA wasn't given enough money to properly develop the airborne launch platform. Never mind the fact that it would have replaced the boosters and fuel tank, something that would have paid off big-time in the long run.