Somebody asked me this question today knowing of my interest in the case and I could only answer that I didn't know. What I could say however, having thought about the delay for some time, was that it most probably indicated 1 of 2 things, perhaps both.
No longer in an inquiry can the inquirer make his or her finding without consideration of the harm that might befall any person that might feel they didn't have a right of reply against pointed decisions that arguably might make those reading the report, the public, conclude that the had acted improperly in some way. In other words one might be concerned that Milton Weir in the Bain case could object to claims that he allegedly planted evidence at some time, despite there being Court cases in which he had been able to defend himself from that position. Ian Binnie pointed this out, that the allegations he levelled against some involved in the police investigation into the Bain murders had been defended by those police directly years before his finding that David was factually innocent.
For example ex Detective Sergeant Doyle might still wish not to call a strip search a strip search. As to this most people would agree that common sense prevails. However, it may be the case that correct names put to situations such as David Bain being strip searched and no scratches being found on his chest, or that his father had blood on his palms despite allegedly not being the killer of his family, then himself - are weighing on completion of any finding that David is innocent on the balance of probabilities.
I agree that people should have a right of reply to accusations levelled against them but would argue that none, or few, allegations as to the misconduct of police in the case have not already captured responses. For that reason is underlined why the application for compensation is not decided in a Court is foolish at the least, calculatedly contrived at the worst. No Court case would be allowed to be held in secret, such as it is, most, if not all of the case for compensation would be heard by the public. More on that later perhaps.
To the second probability. Very obvious to most keenly looking for logic are the forensics of this case. The forensics have continued to grow in favour to a clear perspective that David couldn't have killed his family, while plainly his father could have- and did. Much like individuals involved in the investigation and now criticised for their performances are held to demand another right of reply - so too the forensics. The Crown and police can't escape the forensic clarity in the case of the Bain murders and the suicide of Robin Bain. But they will try, as I believe they do now. Anything to delay, anything to keep secret the extent of the injustice against David Bain, his mother and siblings.
Those can be the only reasons why this inquiry is on extended time, and why it hasn't been open to public scrutiny from the outset.