"Much of the correspondence does indeed reflect the Prince of Wales's most deeply held personal views and beliefs. The letters in this case are in many cases particularly frank.
"They also contain remarks about public affairs which would in my view, if revealed, have had a material effect upon the willingness of the government to engage in correspondence with the Prince of Wales, and would potentially have undermined his position of political neutrality."
In other words, by exposing the fact that he was not behaving neutrally, people might think he wasn't neutral. And so transparency is vetoed to protect hypocrisy, to allow a member of the royal family to deny in public what he is doing in private.
Only in Britain would such deference to an unelected inbred trump basic constitutional principles. Bring on the republic!