If I somehow legally obtain your diary and publish juicy exerpts and facts from it (publishing all of it would be restricted by copyright), then you may have grounds to sue me for violations of your privacy. If you're dead, your heirs can't do that, they can protect their rights (e.g. the inherited copyright), but not your privacy. It's similar for defamation; after you're dead, defaming you does not violate the rights of anyone - anyone living, that is.
In my local jurisdiction, I would not necessarily expect that to be protected by privacy law even if I was living (you may have grounds, but good luck!) - and it would be protected by copyright either way.
the rules around publishing (and reading) private diaries are the same for dead people as they are for living people, aren't they?