Because a new supported OS needs new builds, probably modifications and a lot of testing.
Fedora has a new release about twice a year (and only the latest two are supported by the Fedora Project). As it stands Plesk doesn't even support Fedora 7 yet, and Fedora Core 6 will be EOL'd by the Fedora Project in december. I'd say Fedora is moving way to fast for use as a stable hosting platform, but that's for you to decide.
Fedora is a nice distribution, but for serious hosting I'd look into using something like RHEL or CentOS (7 years of security updates instead of just over a year).