From the Document: "The United States is making significant efforts to pursue a strategy that ensures continued access to space for national security missions. The current strategy is embodied in the National Security Space Launch (NSSL) program. The NSSL supersedes the Evolved Expendable Launch Vehicle (EELV) program, which started in 1995 to ensure that National Security Space (NSS) launches were affordable and reliable. For the same reasons, policymakers provide oversight for the current NSSL program and encourage competition, as there was only one provider for launch services from 2006 to 2013. Moreover, Congress now requires DOD to consider both reusable and expendable launch vehicles for solicitations after March 1, 2019. To date, only expendable, or single-use, launch vehicles have been used for NSSL missions. The NSSL program is the primary provider for NSS launches. Factors that prompted the initial EELV effort in 1995 are still manifest--significant increases in launch costs and concerns over procurement and competition."
CRS Report for Congress, R46211
Congressional Research Service: https://crsreports.congress.gov/