Largely because the US is the only country (AFAIK) to have produced such a complete and thoroughly researched set of guidelines (mainly 'cos they have a big population of patients to test things on and a big population of shrinks and medics to do all the necessary testing). The APA's DSM-IV is basically the international standard for diagnosing mental illness; (Some) other countries base their guidlines on DSM-IV, just issuing overriding documents where they happen to significantly disagree on something.