JPAS to DISS - the transition process

WHY -The U.S.A. government is transitioning from JPAS to DISS for its security clearance and public trust determinations in 2016 and 2017. Once DISS is fully deployed, it will replace the Joint Personnel Adjudication System (JPAS). It will serve as the system of record for personnel security, suitability and credential eligibility management for all military, civilian, and Department of Defense (DoD) contractor personnel. /// Note JPAS was only used in DoD, the US Gov is much larger with many more organizations. Scattered Castles is what the rest of the gov uses. Why is DoD making a replacement for JPAS, when we should be adopting Scattered Castles? SC has been operational for over a decade....why the cost$$!!??