The agency tolerated CentOS but was never happy about it. The fact that it was directly derived from RHEL and could be expected to get timely security fixes probably helped make them comfortable with it. The agency is also concerned about Python2 remaining the default in both CentOS 7 and RHEL 7. Red Hat has committed to back porting security fixes for Python 2 proper, but that leaves a large universe of add on packages that might not get patched. Thus the move from both CentOS 7 and RHEL 7 to RHEL 8.