Nobody decided to do it yet, simple as that. It is not a nice task, you will have to patch the compiler until it passes its test suite, than you have to rebuild all existing packages using this compiler (only in case of an ABI change), if something fails probably patch the package build and/or the compiler build.
It just takes a lot of effort and conda-forge is just like Fortran-lang entirely driven by volunteers. Helping hands are always welcome!
The workflow for a compiler update on conda-forge is outlined in