I like you would prefer if accident investigation was just an aviation technical issue but unfortunately there are others who want to stir the pot for there own ends, the AAIB is not and should not be put under pressure from anyone to publish until they have the report as close to the facts as posable.
This is inconvenent for politicians , journalists and lawyers who all have agendas that are unlikely to be in the intrest of flight safety.
My view was also held by late wife (PPL/IMC/Night) who started her career as a journalist, in her opinion air accident investigation was far too important to be swayed by any of the above mentioned professions by rushing out a report and getting some of it wrong or having it written in a manor that can be misinterpreted.
My guess is in the case of the Glasgow helicopter accident much political pressure was applied and just as the AAIB had announced a publication date new evidence came to light, the resulting delay needed for reevaluation of the facts resulted in a missed publication deadline, naturally that women from the SNP was all over this for self promotion purposes and the AAIB ended up looking less than professional............ Well it is the last time they will do that !