Where there is blame there is a claim !
The answer to your question as to why it takes so long is that as soon as the report is published every anblulance chasing lawyer with even half an interest in the case will be looking to use the report to get money and/ or making a name for themselfs by exposing a "cover up or whitewash" by the authorities.
This results in the AAIB being very reluctant to publish anything definitive until they are 100% sure they have the all the facts in place.
The pain for those bereaved by this tragic accident is prolonged by the modern requirement for so called closure and the insistence by the media that they have to know all the facts before this can happen.
As for the Glasgow helicopter crash and the comments by that woman from the SNP, she as a lawyer should know how long it takes to compile a reliable report and her comments are just political grandstanding at the expense of the pain and suffering of those directly involved.
The unfortunate fact is that no amount of reports will bring back the dead and pursuit of the truth as the lawyers and media see it is not helping those who have lost loved ones move on with their lives and a rushed half baked report will just make matters worse.