Currently, all OCP related data is either from quay.io, redhat.com or openshift.com - but the cinncinnati graph data is hosted in github.com.
Some customers do not allow openings to github.
From a RH Consulting standpoint it makes it easier to argue for a firewall opening towards any of the redhat related domains, compared to github.com

The documentation is already referencing api.openshift.com, as seen here:
https://docs.openshift.com/container-platform/4.12/updating/updating-restricted-network-cluster/restricted-network-update-osus.html#update-service-graph-data_updating-restricted-network-cluster-osus

But oc-mirror is still using github.com

In addition, it would be nice to define a custom url, if you have downloaded the graph data and put it on an internal server.