Why is .gov only for US government agencies?

Wondering if some of the long-time list members can shed some light on the question--why is the .gov top level domain only for use by US government agencies? Where do other world powers put their government agency domains? With the exception of the cctlds, shouldn't the top-level gtlds be generically open to anyone regardless of borders? Would love to get any info about the history of the decision to make it US-only. Thanks! Matt