Because it isn't just getting a body of code certified once.
Someone has to maintain that specific code and either upstream or backport important changes which require a re-certification, neither of which LibreSSL is interested in helping with.
For example, create a fipssl project which is patches to a specific version of LibreSSL; take the result (LibreSSL v1.x + fipssl v1.2 = libfipssl v1.2) and then get that certified. Companies/contractors which need FIPS certification then use libfipssl v1.2.
The fipssl maintainers then just have to track LibreSSL changes until their next release, which (for a series of small, maintainable patches) shouldn't be too difficult. The grsecurity project has been doing the same thing with the Linux kernel for years.
Well, need & money, both of which Red Hat has. I am unclear how a company that maintains an enterprise distribution and writes a lot of code cannot find the folks needed to do FIPS if it is that important for their government contracts.