Because it would introduces implementation details into barclamp-provisioner which only barclamp-pacemaker should know about (e.g. the role name `pacemaker-cluster-member`).  `provisioner` is a core barclamp but we are trying to keep `pacemaker` as non-core. (There is currently a weak coupling with `barclamp-crowbar` but we would like to eliminate that in the future, e.g. via some kind of plugin mechanism.)