Where are you getting that about a "server OS.
Lets take RHEL for example :- to ensure that it is a stable platform for server usage it runs a kernel roughly 12+ months old, has security fixes backported and RH ensures the ABI isn't broken by every update. This allows businesses to have a longer server OS lifecycle without sacrificing stability or security.
Fedora on the other hand is a development distro, running the latest kernels and therefore support for very recent hardware, the latest versions of applications etc
The RHEL target audience is the business server market, Fedora is not.