Do you know why? Is it because of the docker hub pricing changes?
I found this discussion on the mailing list but didn't see a reason why: https://lists.jboss.org/pipermail/keycloak-user/2019-March/0...
https://lists.jboss.org/pipermail/keycloak-user/2019-March/0...
Mostly I think the answer is just that it's a Red Hat project and Red Hat wants to use their ecosystem.
Having said that, I know there has been some falling out between RH and Docker some time ago, which was one of the reason RH ended up creating Podman.