We cannot speak for the government or other agencies but resources need to be allocated to where they are needed most.
Contemporary search and rescue tasks where lives can be saved are the priority rather than legacy events.
Additionally, the objective of an aviation safety investigation is defined by the International Civil Aviation Organisation as:
The sole objective of the investigation of an accident or incident is the prevention of accidents and incidents. It is not the purpose to apportion blame or liability.
It is obvious from the evidence gathered by the aviation safety investigation agency of the time that the causes to the VH-MDX accident were rather clear and that safety related conclusions could be made.
Accordingly, the objective of the safety investigation agency has been achieved and location of the aircraft whilst desirable was not required to achieve the objective.
The current Australian aviation safety investigation agency, the ATSB (Australian Transport Safety Bureau), is not funded to investigate every aviation accident. Accidents that have a predicted larger safety outcome to aviation have more resources allocated than those accidents that have less perceived safety outcomes.