I know the police do an investigation, which seems to be a prerequisite for the resulting coronial investigation, and that seems to be as far as it goes. Like you say, it doesn't take three years and try and reinvent the wheel.
Sorry but a quick search of coronial inquests results in Victoria and the first one took 4 years to deliver being complicated, the next 5 between 2 years to 6 months depending on complexities. Quite simply, no one witnessed the crash, there was no hard evidence of causality only speculative evidence. In these cases the investigation will hold out for any chance of further evidence that could change the outcome and then release.
Comparing to road investigations is unfair. In most road collisions there will be lots of witnesses, cameras, skid marks and fairly straight forward analyses can be done fast and efficiently. With air incidents, especially this one there is lots of speculative evidence based on past habits, however no hard evidence of cause can be ascertained. You can only rule out what did not happen and then report on the most probable occurrence. The only hard evidence, the aircraft crashed at high speed, it was not survivable, the pilot was healthy enough to rule out incapacitation and RADAR plots were indicative of erratic flight path. The rest is speculative as to what was occurring and why. With such limited evidence, put the file on the shelf and wait for any chance of additional evidence such as a reluctant witness, some lucky video capture from the edge of frame, some go-pro or similar that washes up from the crash, etc etc....