Dunno why he's not getting hired but I don't think calling plays should have much to do with it.
John Harbaugh was Andy's special teams coordinator. Never called plays. Great coach. Doug Peterson didn't call plays under Andy. Good head coach. Even a lot of coordinators who did call plays before they became HC's didn't continue calling plays as the HC.
Game planning, leadership, teaching, in game coaching... all are far more important for a HC. Just hire an OC to run plays like most do.