NASA's unmanned X-34 shcramjet demonstration sub-orbital aircraft is air-launched from Orbital Sciences Corp.’s L-1011, a commercial jetliner originally modified to carry the company’s expendable Pegasus launch vehicle. I understand that shcramjet engines need to be boosted to supersonic velocity before they start to work, but I am wondering, "why they are air-launched rather than boosted using a rocket from the ground?"

I suspect it has something to do with reducing vibrations and g-forces on instruments and sensors, or concurrently demonstrating reusable launch vehicle technology, other than the Pegasus, which is considered expendable. I am wondering about the cost difference as well.