All the investigation of the crime was conducted from Santa Barbara: all the search warrants, interrogations, witness interviews, the first arrest and observing her in jail, the likely cellphone/computer searches and other monitoring, etc, etc.
Those detectives and the prosecutor they worked with have been with the case from the first day they interviewed A, and she refused to co-operate. They know the case inside and out.
Less than two days after finding her body, Wayne County investigators contacted Santa Barbara police, when they realized it might be M. They had no info themselves, about how she came to be there.
IMO, the FBI, working with Santa Barbara detectives, immediately took over the case from Wayne County.
It makes no sense for the Wayne County prosecutor to file charges: they were not involved in any way except finding the remains and alerting Santa Barbara. They would have almost nothing to contribute to a trial. All the evidence in court will come from Santa Barara, and it was the SB Prosecutor who was key to deciding what charges to lay (premedited murder), and how to bring the evidence to trial.