It's due to unavailability of direct A class crew (even Crew Lobbies have categories :0) between Rourkela and Gomoh.
ROU lobby takes charge from ECoR to ROU. Link was missing between ROU - GMO. CKP Lobby is Cat. B so cannot operate premium passenger trains. Only viable option was ANR lobby. For reference, Anara is the largest lobby in terms of crew strength in SER.
ANR lobby mutually completes the link in two shifts i.e. ROU-ANR and ANR-GMO