I think it’s probably because for server usage, RHEL/CentOS is used significantly more than Fedora (with its shorter supported lifecycle), and Fedora is essentially the upstream for shakeout testing prior to inclusion in RHEL/CentOS, so hardening and security technologies – e.g. SELinux, fstack-protector, etc. – are very close. RHEL/CentOS 7 was based largely on Fedora 19, and newly-released RHEL 8 is based largely on Fedora 28.