If Spedding has been found guilty of the NSW charges, or settled on some kind of plea deal, the girls may have thought that was enough for them.
They may have been satisfied with the outcome, and felt they had been through enough trauma and interrogation on the stand ... did not want to proceed any further.
Or the charges may have been withdrawn pending further and ongoing investigation. There is a strike force that was formed to investigate the Victorian matters ... perhaps the investigation has blown up to be bigger than Ben Hur. More incidents and adults/children involved than originally thought.
The police/prosecutor cannot just hang on to a person who has been charged. Everyone has a right to a 'speedy trial'. Charge them and try them, or withdraw the charges and re-charge them later when fully ready.
The right to be tried without unreasonable delay is recognised as a human right under the Victorian and ACT human rights legislation.
Australia - Criminal Defense Wiki