The fixed colormap is a per-scene global setting that normally does not need to change ever during rendering of a scene so it's easily shoved aside into a static uniform buffer.
Having to change this buffer for inconsequential stuff should be avoided, especially when there's other uniforms that are just as good to hold these values.
These files are not part of the actual renderer but part of the system code.
This means, for separated modern and legacy GL renderers, there still will only be one set of this, unlike everything else.