I'm not sure why. I have some personal knowledge of this issue, because I actually tried to argue it in the Federal Court as an individual back in the mid-1990s, when I put in an application concerning how the government sets fees. Essentially they charge a nickel or a dime or a quarter for photocopies. I was told there were 1,700 documents, 1,600 of which were cabinet confidences.
Two and a half years after complaining to the then Information Commissioner, I got a note back from the commissioner saying they were sorry it took so long. I thought there was a loophole there. I took it to Federal Court. The judge said they weren't going to look at it.
It seems to me that the government essentially brought in the equivalent to section 39 of the Canada Evidence Act. They brought the certificate out of an abundance of caution. It was to make sure nobody would ever look at it and to kill off the complaint.