Very simply, because Congress determined that NASA has a sufficiently distinct mission and procurement requirements from DoD to justify having its own FAR supplement. One could think the mission of NASA is somewhat related to that of DoD and should be subject to the same set of procurement rules. However, the Veterans Administration might also be seen as "somewhat related" to DoD, but it too has its own FAR supplement. In other words, the true nature of an agency's mission may justify the agency being able to shape its procurement policy in a way that best suits its own, unique mission requirements. Of course, all agencies are subject to most of the same policies and procedures contained in the basic FAR.