Because the only secrets a government should be allowed to keep are the ones where a loss of secrecy would render the whole point moot. So, for example, when pl

Because the only secrets a government should be allowed to keep are the ones where a loss of secrecy would render the whole point moot. So, for example, when planning a wartime offensive against an enemy the military and by extension the government must have the privilege of keeping their plans a secret, since to do otherwise would make the plans useless. Once keeping the plans secret no longer serves that purpose they should be made public. Diplomatic cables do not meet this standard in the general case. They are a log of a government's interactions with another government, and thus need to be public so that the people can confirm it is working in their interest, and not conspiring to implement a global copyright regime at the behest of Big Media, or aiding a foreign government in oppressing its people, or sending detainees to foreign soil with the understanding that they'll be tortured, or trying to overthrow democratically-elected leaders, or any of the other reprehensible things that the American government is doing literally while I type this, right now.