Probably the same reason many things are contracted out, UG mining is a specialised business and is best left to those who specialise in it at a fixed price per tonne, RVR specialises in finding, purchasing and restarting failed mines , doing the groundwork and running the plant.
Armidale will be equally attractive to whoever employes them and if a contractor does his job properly will be there for the life of the mine anyway...
Cheers Whisky